First, place your things that MUST go in certain places. Like, the computers have to go wherever the internet line drops. The teacher computer must go where the projector hookup is. And I think the whole group meeting area must go wherever your main teaching board/projector/whatever-you-use-a-whole-bunch-thingie is.
After that, create a list of areas you want to "create" - I want to have a library in the corner, for example, or a guided reading group table near a bookcase, etc. Set up those areas where you can, and just fill in the holes.
And yes, like a PP mentioned, pay attention to the flow of your room, and make sure there is enough room for the kids to move their chairs back from their desks.

I usually end up putting mine too close together!