In an effort to assist those of you guys looking to record your ATIS with a bit of a more professional flavor, I'm going to include some tips for how to properly translate the METAR to voice format and make it sound a bit more realistic.

I had taken the time to write some detailed ATIS policies for airports in Chicago during my time there and have spent a considerable amount of time listening to actual ATIS recordings and reading up on how to properly record them.

This is just to help those of you guys out, like myself, who strive for that ultimate perfection while you control . I've tried to dumb this down a bit for VATSIM applicability - feel free to review the links I've posted to delve further in depth at your discretion!

Let's start by taking an example ATIS to work with:

KLAX 281750Z 24006KT 10SM SCT250 33/M03 A2995 RMK AO2 SLP141 T03281033 10339 20206 51003

First portion of the ATIS, a decoding of the METAR and how to pronounce the parts of the coding while recoding. I have broken this down into 12 parts to explain a bit about each.
 

1. The beginning of your ATIS should begin with the official facility name of the airport you are working, followed by the current information code. KLAX 281750Z 24006KT 10SM SCT250 33/M03 A2995 RMK AO2 SLP141 T03281033 10339 20206 51003.
   
   For instance, with Los Angeles, it should read as follows:
   
   "Los Angeles International Airport information CHARLIE" - the full facility name can be found on the charts of the airport as well as in an airport facility directory or airnav.com (or some other airport information sites). Replace CHARLIE with the current ATIS code, whatever that may be, via stating the appropriate letter in the aviation phonetics format (ie: ALPHA, BRAVO, CHARLIE, DELTA, etc...). For the purpose of VATSIM, it does not make a difference what letter you begin on or when (when you log in to control) - just ensure you follow up with the next letter in alphabetical order for the next update.
    
2. Include the time of the observation in zulu. KLAX 281750Z 24006KT 10SM SCT250 33/M03 A2995 RMK AO2 SLP141 T03281033 10339 20206 51003
   
   "one-seven-five-zero zulu" - 1750z - pronouncing each digit separately at the time of the observation. The date (in the example above 28 ) should not be recorded in the ATIS. A routine observation is the normal time the ATIS is updated (for LAX, this is 50 minutes after each hour). A special observation occurs during adverse weather conditions and may be updated a number of times between routine observations. This should be stated as such when applicable "XXXX zulu special".
    
3. Wind direction comes next, gusts shall be reported after wind as “Gusts XX” when applicable (replacing XX with the gust in the METAR). KLAX 281750Z 24006KT 10SM SCT250 33/M03 A2995 RMK AO2 SLP141 T03281033 10339 20206 51003 
   
   "wind two-three-zero at six" - Due to magnetic variation, here at ZLA we subtract ten degrees from the wind direction within the METAR to be more exact (as in this example). Note that there is only ONE wind. Stating "winds" is incorrect and a common mistake made by many. Gusts would be included right after the wind as "gusts XX" (not "gusting") replace XX with the Gust in the METAR where applicable (indicated by a G after the wind direction and speed).
    
4. Visibility is next. KLAX 281750Z 24006KT 10SM SCT250 33/M03 A2995 RMK AO2 SLP141 T03281033 10339 20206 51003
   
   "Visibility one-zero" - Note this is ASSUMED to be in statute miles ALWAYS (SM) - no need to include "statute miles" in the recording itself. If visibility is listed as something like 1/4SM or 1 1/2SM it would be read, respectively as "visibility one-quarter" or "visibility one and one-half". If VVXXX (Vertical Visibility) is noted in the METAR, then “indefinite ceiling XXX” shall be included after the visibility is issued, replacing XXX with the height in feet. When the visibility is less than 7SM, included in the METAR will be the reason why (an obscuration) which should also be stated directly after the visibility in the ATIS (ie: "visibility five, mist"). There are many other factors for visibility beyond the scope of this discussion as well, but you can read up here on other types or reported visibility which can also be noted in the METAR here: http://www.met.tamu.edu/class/METAR/metar-pg7-vis.html 
    
5. Clouds are next. KLAX 281750Z 24006KT 10SM SCT250 33/M03 A2995 RMK AO2 SLP141 T03281033 10339 20206 51003
   
   "Two-five thousand scattered" - report as applicable beginning with the lowest cloud layer. Few cloud layers should be read as “few clouds at XXXX.” Scattered, broken, or overcast cloud layers should be read as “XXXX scattered/broken/overcast.” The lowest broken/overcast layer constitutes a ceiling and must be reported as such within the ATIS, “ceiling XXXX broken” or “ceiling XXXX overcast.” Replace XXXX with the altitude in thousands of feet (EX: three-thousand or six-thousand five-hundred). If CLR is noted within the METAR, then issue the cloud report as “Clear below 12,000” as this is the maximum height the weather equipment is considered to be valid at Los Angeles International Airport (this is noted by the type of weather reporting equipment at the end of the METAR).
    
6. Temperature and dewpoint. KLAX 281750Z 24006KT 10SM SCT250 33/M03 A2995 RMK AO2 SLP141 T03281033 10339 20206 51003
   
   "Temperature three-three, Dewpoint Minus three" - Note, again, that the digits are pronounced individually, not as whole numbers. This is for the sake of clarity. If you note an M in front of something, that is pronounced as "Minus" such as in this example METAR report.
    
7. Altimeter setting. KLAX 281750Z 24006KT 10SM SCT250 33/M03 A2995 RMK AO2 SLP141 T03281033 10339 20206 51003
   
   "Altimeter two-niner-niner-five" - As above, each number pronounced individually, without the decimal as it is a given.
    
8. The next portion of the recording shall include the advertised instrument approach(s) in use at the airport. The following list is an example of how this may be recorded:
   
   "Simultaneous ILS approaches in progress to runways 25L and 24R" 
   "ILS runway 9 approach in use" 
   "Localizer and visual approach runway 27 in use" 
   "V-O-R Alpha approach in use, circle to land runway 27" 
   "RNAV runway 6R approach in use" 
   
   Note that all runways shall be read out as individual numbers while recording (EX: 25L – Two-Five Left). Instrument approaches should always be listed prior to visual approaches in use - when applicable.
   
   Also, when applicable, LAHSO (Land and Hold Short Operations) shall be stated. "Land and hold short operations are in effect"
   
   Be sure to include the applicable LAHSO runways in use, stating the available landing distance for the LAHSO runway. (ie: "Runway 15 arrivals may be asked to hold short of runway 8 for landing traffic, available landing distance on runway 15 six-thousand, five-hundred feet"). The available landing distance is very important as the pilot needs to know the stopping distance he has and whether or not he/she can accept the restriction. 
   
   Additionally - when applicable, include RVR (Runway Visibility Range - during low visibility operations - ie: fog/mist) and any braking action reports - although this should also be issued to the pilot from approach and the tower in the landing clearance. 
    
9. The departure and landing runways in use.
   
   "Departing runways 24L and 25R" 
   "Landing and departing runway 27" 
   
   Pretty self-explanatory - again, pronounce the runway numbers individually for clarity. 
    
10. NOTAMs (Noticed To AirMen) that may apply to your airport.
    
    "Notices to Airmen, runway 25L closed" 
    "Airport closed to traffic pattern operations"
    "Parallel approaches in use between Los Angeles International Airport and Hawthorne Airport" 
    
    Anything else you can think of that may apply to the airport you are working... these are just examples. 
     
11. The fifth section of the ATIS shall consist of the following to conclude the recording:
    
    "All departures contact [CURRENT CLEARANCE/DELIVERY POSITION AND FREQUENCY] prior to taxi" (or something to this tune so the pilot knows who to call for their clearance).
    
    "Read back all runway hold short instructions [and runway assignments]"
    
    "Advise controller on initial contact you have information [ATIS CODE]" (or again, something close to this tune works fine).
    
    This way the pilot knows to call you with the information. (and gives you a reason to instruct the pilot to listen to the ATIS when they don't call with it). 
     
12. LASTLY (finally right?), and if you remember nothing else from this lengthy article, I ask you to PLEASE REMEMBER THIS PART!
    
    A pause must be included to clearly define the beginning and end of the ATIS for a period of three to five (3-5) seconds at the end of the recording to prevent an “endless recording” when pilots listen to the frequency (or what I like to refer to as the "endless ATIS recording of DEATH". 

So, to sum up above, here is a typed version of what would be read for the ATIS recording on this METAR.

"Los Angeles International Airport information CHARLIE, one-seven-five-zero zulu observation, wind two-three-zero at six, visibility one-zero, two-five thousand scattered, temperature three-three, dew-point minus three, altimeter two-niner-niner-five, simultaneous I-L-S and visual approaches in progress to runways two-five left and two-four right, departing runways two-five right and two-four left, parallel approaches in use between Los Angeles International Airport and Hawthorne Airport, read back all runway hold short instructions and runway assignments, all departures contact Los Angeles Tower one-two-zero point niner-five prior to taxi, advise controller on initial contact you have information CHARLIE (pause for 3-5 seconds)”. 

All can be read within a period of about 30-45 seconds on average!

I also took the liberty of creating an ATIS quick reference table:

ATIS QUICK REFERENCE TABLE
 

Keep in mind your ATIS must be in compliance with the VATSIM EC Global ATIS Policy in which it cannot exceed 1 minute in length (so, practice your recordings to get them done quickly!) and that the textual ATIS should still be in compliance with ZLA's policy found here:http://www.laartcc.org/operating_procedures/36/Requirements+for+Controllers+ATIS

I have received many wonderful comments from pilots on having a clean-cut and professional sounding ATIS and I wanted to provide this information that has helped me attain this to you all to help further advance the professionalism of ZLA. 

I hope this article helps some of you out! Feel free to comment and add in on anything I may have missed! Again, just trying to provide some assistance and further insight on the matter. Finally SX has written an excellent article about Decoding METARs.