From my own experience, I've found that a really good way to have vivid hypnogogic imagery is to imagine that you're playing football with someone. When I do this, the other person tends to kick the ball right past my head and I get a really clear image of it. Just thought I'd post it here in case some WILDers find it useful. If you have any success with this let me know, it would be interesting to see if it works for other people too!