Great advice so far. I guess I can throw in my .02 cents.
Make sure you use electrical conduit. It has a softer corner so that cable does not bind. Using real PVC will give you a 90 degree corner that cable will get stuck in.
Do as much pre-wire as you can and then run the conduit. It is amazing how much wire you can run to a room and still need more.
For each room I ran 2 RG6 cables (receive and transmit) along with 1 Cat5e cable to a phone/ethernet drop. I also ran another Cat5e drop to each location where I thought a control box might go for the built in audio/video distribution. This might be an ir-reciever or a control panel or a simple volume control.
I pre-wired a living room 5.1 system. I also have two speaker drops to most of the other rooms including the garage.
If you run the cable yourself do it after the electrician is done (they love it if you already have nice holes drilled for them). Don't run anything in parallel to the electrical runs and if you have to then only for a foot or two. If you have to cross an electrical run then do it at 90 degree's.
Here are some other facts that might be useful or not:
1)I did all of my own pre-wire. The electrician didn't have a clue so I just did it myself.

2)I have a central cabinet in my dedicated ht (basement) that has all the audio/video components for most of the house.
3)I ran a 2" conduit from here into the attic.
4)I ran a 2" conduit from here to the projector location along with a bunch of other wire. S-video, dvi, rg6, cat5e and component.
5)I have an ump box that contains all of my telephone and ethernet connections. It also distributes some of my composite video signal.
6) I spent a lot of time figuring out where speakers would go and IR receiver's and ethernet/telephone (wireless is making this much easier), etc...
7) I used 1000' of Cat5e, 1400' rg6 (belden 1694A), around 700' of 12 gauge in-wall rated speaker cable.
Sorry for the long winded ramble.
Good luck.
Paul.