Okay, if you’re not level 70 or not looking for a raiding guild then you can probably move on to Step 1.  If you are a level 70 and you are looking for a raiding guild then this step is a must.  Also, even if you don’t fit one of those two categories, this step probably still has some good advice.

First of all - do an inventory on your gear and make sure you understand where you are.  Is your gear appropriate for your class?  Are you under-geared?  What about gems and enchants?  If you’re looking to get into a raiding guild and hit SSC, but you are missing enchants and you have inappropriate gear - then you most likely will be turned down.  Just make sure what you have is a fit for what you are looking for and be prepared to talk about your upgrade plans.

I’m gonna assume you know your class and role and are proficient.  If not, be realistic about the type of guild you are looking for.

Make sure your talent spec makes sense.  If you’re not sure on this one, there are plenty of resources out there to point you in the right direction.  Need more help?  Trying searching for some bloggers within your class.  The WoW community is rich with insightful and helpful bloggers of every make, model and color. 

Lastly - grow up!  Of course I don’t mean you, but there are lots of, forgive the language, asshats out there.  If you are one of them, then you can probaby stop reading right now.  My guess is you bounce from guild to guild every month or so, and are fine with that.  If that isn’t you, then here’s my simple suggestions:

• If you are a fan of leet-speak, instant message shorthand or swearing, stop.  Now.  Most organized, well run, mature guilds will not accept applications written this way.  Correct grammar, along with the ability to communicate effectively in the language of your server, is generally a good thing.
• Learn to show a little humility.  You may think you are Elune’s gift to raiding, but believe me you are not the only good player in the game.  If you come off as overly arrogant, then what that tells the guild you are applying to is that you are simply out for yourself.  Not only will you probably leave as soon as you get what you want if not before, but odds are you won’t be stepping up to help others in the guild.  This is another sure way to get your application rejected (from the good guilds at least).
• Be realistic in your expectations.  It’s okay to convey what you are looking for in a guild in no uncertain terms.  Just realize that are not going to be the only person with expectations. 

Before anyone gets their shorts in a bunch and comments to tell me I’m discriminating against younger players, let me stop you right there.  I’ve grouped with players in their young teens as well as grandma’s and grandpa’s.  Believe me that maturity is not limited to older players.  Likewise, age appears to be no remedy for stupidity.