I might be going against the tide on this one, but I swear: letting one's mind drift a little can be good for WILD. The key is staying aware whilst ones body falls asleep, correct? So, if you can manage getting your body to stop sensing things, etc... it really doesn't matter WHAT you're thinking about, right?

I think sometimes certain thoughts allow the body to fall sleep more quickly, but in reality, as long as you're conscious through the transition and aware enough to realize you're dreaming, it should be okay. Just remember, you're trying to get your body to go to sleep, and not your mind!