I am unsure if this has been posted, although I am guessing it might of been. But it seems that there is a way to download a little faster on Xbox Live. [Or atleast make it seem that way]. After confirming download, wait a couple of seconds (depending on your connection speed) until it gets to be anywhere 5% or above [just to be safe]. Then cancel your download. Xbox will take you back to the download list and there will be a little check next to the demo or trailer (or whatever) you were dowloading, even though you didnt finish the download. Simply click on it again to re-download it and confirm the re-download. It will start up again and should almost instantly jump up to 50% or above. To which it will continue downloading from there.
I really only reccommend this for larger files since the smaller files download at a decent speed itself. I've done this with the Quake 4 demo, Condemed Demo, and Fight Night 3 Demo, as well as on another xbox system I have done this with larger high def movie and game trailers.
Again, I apologize if this has been posted before, if not...
PS: It seems to me that the stuff must download in a large initial package, then smaller packages afterwards. If you try this method later on, say in the 60% or above range, it usually bumps back down to a lower percentage the next time you start it up [like around 56%].