Take Valeria root with a small amount of milk (heated with a spot of nutmeg if you prefer) prior to settling into bed.
Also, be sure to train yourself - humans can be made to be creatures of habit rather easily. The key is to wake up at the same time every day - NOTE that this means actually rising out of bed, not simply remaining awake whilst lying in bed.
A routine may not dramatically improve things, but it is a help, if even for the source of habit.
NOTE: Taking substances (whether they be legal or illegal) for the sake of sleep will most likely cause a level of dependency. I've had friends that have been unable to sleep without taking a hit of marijuana due to them almost always smoking it immediately before retiring. Sex or masturbation can easily fall into this category also.