I do NOT envy insomniacs, and it worries me how many I've met.
Normally I can get to sleep without too much trouble. Hot weather and caffeine past noon or so are the two most likely culprits if I'm having trouble.
There's a couple of things I can do when I think I might need some help. Drinking warm milk or having an orgasm can speed things up. Having a fan on if it's warm enough helps too, and years ago I used to play soft music when it was too cold for a fan.
Being consistent on when you go to bed can help your body know when to start winding down, or so I've heard.