Don't let players use any material you don't a)own or b)have easy access to.

I'm a high school player/DM, and one of our players owns something like 50 books (mostly 3.5 supplements). Whenever there's a new campaign he whines about playing only out of core (which is all I have) and always wants to play almost entirely out of those books. The main problem is that some of those classes, feats, and spells are completely unbalanced when compared with core, but telling him that has little effect since core doesn't really balance with itself either (just compare, say, baleful polymorph to flame strike, or fighter to wizard, and you'll see what I mean).

Also, try to build stuff based on your group. If there's a barbarian PC in the party, take his Rage and ridiculous Strength (if any) into account for encounters and checks. If the players aren't too great with roleplaying, don't push them, just give them a kick-in-the-door game. Of course, you shouldn't base what you do exclusively on what your players say, or you'll end up like me, unable to run anything without arguing over rules which players know better than I do.

PS: If you already know this and this post was a waste of time, I'm sorry.