There are many things to do to minimize the chances of it happening.
First, make sure the contrast is not set too high. It should never be set beyond the halfway mark. Use a calibration disk such as DVE, Avia, or even the THX optimizer on some DVDs to set brightness and contrast properly. Setting contrast often involves switching back and forth between brightness and contrast settings to get them both right. Some RP tvs even have a "game" setting ... use this if it is available.
Stretching the content so that it fills the screen is helpful. This avoids the letterbox/pillarbox darkened areas that can cause problems.
Not playing for very long times with a static image on the screen (health bars, screen logos, etc.) is helpful.
I have read many recommendations on how long static images and/or letterbox/pillarbox images should remain on the screen. 1:3 is a ratio I have seen ... I doubt this is scientifically proven, but is a fair guideline. (So, if you play for 2 hours with your Halo health graph showing, make sure you use a fullscreen movie for 6 hours afterwards, or as close to that as you can.)
If you play a lot of games or watch CNN a lot, I'd check the burnin status about once/month. You can do it by switching to the DVD input with no DVD in, so that you see the blank blue screen. Look the blank screen over carefully, it there is burnin you will generally see the faint shadow of the offending itsm.