The thing is you want to become awake enough so that your awareness will carry into the dream world, but you also want to be tired enough so that you can actually fall asleep. This is a fine line that varies from person to person. My advice would be to experiment with different lengths of time during your WBTB's. For some people, this is 1 minute; others maybe 10 minutes; and some stay awake for an hour or two.

A while back I posted a thread on how to fall asleep faster. Have it a look for it might be useful.

Good luck!