From my limited experience:
If kings are castled on opposite sides, you usually have to your push pawns to his castle too, it becomes sort of a race.
If the kings are castled on the same side and he's pushing kingside pawns, remember to stay calm, it usually looks more dangerous than it really is, remember they're weakening their king's position.
I realize these are not very helpful, but rules of thumb are usually very limited without example positions.
One way is to try isolate a pawn and concentrate on that one. Like they said above, you can try push a pawn to break up the base of their pawn chain and open a file. Then push pieces through the hole or concentrate forces on any isolated pawn that may have been created.
while they are pushing pawns develop your pieces. then you attack. don't let them close the position.
yeah, i agree with ncubbie - just develop pieces to squares where you won't be chased away, then blow the centre open and attack their uncastled king. The idea is to take advantage of their lack of development. Post examples where you have been squashed by the pawn rush and then folk can give you pratical ideas.
I have been improving my game, but once in awhile I will play a very inexperienced player and end up losing because I can't handle the pawn rush. I was wondering if there are some rules of thumb to follow.