Make sure all of the latest reliability patches are installed from Microsoft. They deal with a lot of the issues. Upgrade your video drivers, too. If that doesn't work, try sleeping and resuming without all of your USB devices attached. If that works, plug one in at a time until you find the culprit.
Also, if you have a Creative XFi card, the latest drivers worked wonders for my sleep reliability. Who knew it was the sound card?
Good luck!