Know what you are doing. If you are going to play some weird rogue/cleric/sorc variation have a **** good reason for doing so (and no i dont mean because you think its cool) and make sure that you can perform as well at your chosen role as a straight build while bringing something else to the table. A bad multiclass (like the one above who i played an adventure with recently) doesnt have a good grasp of any of the classes they are playing and for example couldnt see any danger, had pitiful reflex saves, couldnt find trap boxes or disarm them and so tried to run through traps and survive (somehow) by healing himself.
A good multiclass build has a focus in mind but also attempts to fill the roles of the classes it has. Dips into rogue and monk levels happen a lot because of they shore up one dimensional builds by offering skills/evasion/saves/ac bonuses to classes that lack them. Prestige enhancements are some of the best reasons to multiclass but again the focus should be on what you want the character to do. For example my 13/6/1 rogue/barbarian/fighter build was built as a dps build (warforged power attks, acrobat speed increase, sneak attack and frenzy can be a pretty good beating) but because he also had rogue levels primarily I made sure that he was always geared properly to be an effective thief. Will he be good as a rogue at epics? Hell no! But the end goal was dps not being an effective thief. Proper gear helps a lot and I would definetly recommend 32pt build as they can turn a lame multiclass into something awesome or at least smooth out potential rough edges - unless you for some reason have a slew of +2 or +3 tomes lying around. But thats just my opinion and experience, I'm sure there is more uber out there.