Sound was the main problem we had with our film. Our set was built in a very echoey hall (it was all we could get) and much of the dialogue reverbed too much to be usable. We ended up dubbing the whole thing!
I'd second the notion to do as much planning as possible. Plan every shot, every angle, visualise the whole thing before you shoot it. We didn't do enough of this, and we found we wasted a lot of time just trying to work out where the camera should go for every shot. A bit of a pain, that was.