I don't have very many ideas for how to make yourself fall asleep when you can't, but there are things I try to do BEFORE it gets to that point lol, but then again, I am prone to have trouble sleeping anyway (but I have bipolar disorder so that is probably why).
I've read that you should try to turn off tv, computer, etc. (basically anything that emits light) as early as possible because the light can prevent your body from releasing melatonin (one of the chemicals that makes you fall asleep). That actually seems to work fairly well for me when I can actually force myself to stop using the computer and/or TV a few hours before I go to bed lol...and other than that I try to do all the "typical" stuff they say to do or not do... Avoid caffeine, don't exercise too close to your bed time, etc.
But on the nights I just absolutely can't seem to sleep sometimes I try to lay perfectly still with my arms down by my side and concentrate on nothing but breathing and I'll try to imagine all the stress of the day flowing out of my body through my arms and legs with each breath. Sounds super cheesy lol, I know, but a psychologist told me to try that once and it works pretty well.
And I don't know why, but sometimes I can put the TV on one of the home shopping channels and if they are talking about jewelry they usually do it in such a soft, monotone way and just listening to them puts me into a super relaxed state of mind and then I fall asleep...but I usually only do that on days when I need to go to sleep but I know I don't have to get up early. I think sometimes my mind gets hyper and I feel like I should be "doing something" even though I need to relax and/or sleep. I think for some reason listening to the those people talk makes my mind feel like I'm doing something but it's just boring enough that I can fall asleep. lol