I see all the usual advice was already given so I'll skip right to the weird things that work for me: I lie on my belly with my face resting on my palm so that the base of the thumb gently presses on one of the eyes (right hand on the left side of the face or vice versa). The pressure makes me sleepy almost immediately.

At the same time I think about some detailed but impractical project, anything I'd like to make but cannot immediately start. I think about the tiniest details until I fell tired. The goal is not to try to fall asleep and not to think of anything that would require action/attention, and also not to dwell on anything unpleasant. I usually fall asleep under 5 minutes.