Stalk the job boards for casting calls. You should check this everyday and be ready to audition on very short notice. Sometimes these casting calls are put up a week or less before the audition date.
Decide what persona you works for this particular casting call and cultivate it. What? You thought you just showed up and sold yourself? That's not how it works unless your natural self is bipolar or extremely dramatic. Most reality tv stars inject an element of acting into their behavior. Some actually are nothing like what they appear on tv.
Casting call day. There is no need to get there 10 hrs before the audition starts. There is no need to wear a pink wig and those neon blue platforms sitting in your closet. These gimmicks are good for having the camera pick you up when they are showing who showed up for the audition but can actually work against you once you're inside. As one casting director said to me "I usually ignore the crazies". He also told me that he was biased against those that got there 10 hrs before and those that come in at the last second. If you can't help but fall into those two categories. You have to work harder on developing your persona.
The actual casting process differs based on what type of tv show it is. For things like Big Brother, Real World and I want to work for Diddy. The first stage is usually in semi-large groups where you have to discuss something or talk about yourself.
Be Vocal.
Scratch that. BE LOUD.
This is not the time to work on the manners your parents have instilled in you. Cut people off. Be confrontational. TV is about drama and that's what the casting director is looking for. If you wait for your turn to speak it might never come. That being said, it's very easy to cross the line and appear like you're just talking to shut everybody else up. Make sure you have something to say (this is where developing your character comes in handy). Usually this will get you to the second round.
The second round once again is tailored to each tv program. For things like the Apprentice and Project Runway, your qualifications are more heavily scrutinized. Don't expect to get cast on the Apprentice and have never gone to school unless of course they are doing something like street smarts vs book smarts. You won't get cast on Top Model if you are 5'2 and 180lbs. Be realistic.
In short, the first stage is about personality and what your story is while the second is more about your qualifications.
Now get out there and audition. Can't wait to see you on TV.