Simple stuff, open the window and take a good breath of clean air, get a glass of water, make sure you don't have to go to the bathroom before you go to bed. Make sure it isn't too hot or too cold, that there isn't a funny smell in your bedroom, etc. etc.
Do breathing exercises if you really can't fall asleep. That doesn't have to be yoga or stuff like that, purely breathing exercises. For example the following:
Inhale through your nose, exhale through your mouth. Breath like that slowly while you are lying in bed. This will usually suffice but if you want to you can flex a group of muscles, like, say, your left leg, and inhale while you do that, and then exhale when you stop flexing them, and you can repeat that for your entire body.
To be honest, I do this exercise very rarely as I'm usually tired enough to fall asleep, but if I really can't fall asleep I use this and I assure you it works.
Masturbate? No really, that'll knock you out.
No. Only if you really feel like it and you keep thinking about it, it's better to just do it, but don't if you just can't fall asleep but you don't feel the urge to do it. It'll have the opposite effect.