Just try to keep your mind in a state of calm relaxed awareness. Trying to do anything too much can just get in your way. By focusing on staying calm and being aware of anything you experince, you willl notice the different stages you go through (sleep paralysis, hypnagogic imagery, dream/image flashes and then the dreamscene forming) without getting too drawn into any one, which can make you lose consciousness.

So in short let your thoughts wander, but make sure there's a part of you that is watching them wander so you don't get too absorbed in those thoughts and fall asleep.