I know this one! I know this one!
My son is a natural trasher. He generally trashes my house and I think it's my responsibility to make sure he doesn't trash other people's houses. He's 2 and a bit.
Ok, so, my tips are:
1- believe me if I say they can't come over, even if you have kids the same age. Don't be offended; go to their house instead.
2- put up any special/easily breakable toys
3- if you don't like colours of play-do being mixed, only put out one at a time (etc on things like that)
4- accept that conversation will be a bit one sided and that I'm going to lose track of what was said almost constantly because I have to keep an eye on my son
and, the biggie:
5- please tell me (nicely!) when my son is breaking the house rules or doing something you don't like and please, please, please, feel free to ask him not to do something (ie "We don't do that here" said to him is, for me, the easiest way to hear I've just let him do something not on at your house)
If your friends have problems with no 5, then don't invite them over. It might be that you do number 5 and then number 1 happens - again, if so, don't be offended! It's easier for us to have people over here, especially if I like the parents and want to talk.