First, keep the white level ("Contrast") at a reasonable setting. Then try to mix your programming with both 4:3 and 16:9 material. You should be okay.
Well, I assumed that's what you meant. When watching 16:9 material, have the set in 16:9 mode (that is, for HD and DVD). If you are watching DVDs that are letterboxed only (non-enhanced), then the TV will remain in 4:3 mode while presenting the letterboxed picture. Either way, you want the phosphors to wear as evenly as possible. That's why you should watch a healthy mix of 4:3 programming.