High level games
Well having run a few successful (IMO) high level campaigns make sure you keep challenging the players not only on a physical level ( tough monsters and NPC's ), but mentally. The mental part is important. Sure you have a 15th level fighter who can slaughter his foes by the dozen, but how the hell does this lock work on this door its like a puzzle?? I notice that most high level players quickly get bored with regular combat as with more hit points, more spells, more magic items, etc. fights tend to either last a long time or a very, very short.
So my advice is to make sure one of the aspects of your campaign are tough mental challenges or else decision making challenges that require the PC's to think rather than act.
