Firstly, when I get into bed, I need to relax all muscles. I begin at my feet and think about my toes, my instep, my heel, my ankles, my calves etc and before going from one to the other I make sure that they are all consciously and throughly relaxed. I do all this slowly and for as long as it takes to get it all to relax. I do the same for my hands and arms. Usually for me, these things are already relaxed, although my shoulders and hands are sometimes in need of conscious relaxation, bit by bit. The difference comes when I begin on my face and neck. Think about forehead - relax, think about eyes -relax, think about cheeks - relax, think about mouth (a big one for me) - relax, jaw - relax, neck - get it in a comfortable position and relax. Keep this up until every part of your body is relaxed - no clenching!
Now - when you have complete muscle relaxation it's time to rid your mind of unwanted thoughts that keep you thinking and therefore keep you awake. To do this, I practice a meditation technique of sorts. Firstly I need to stop thinking about whatever I'm thinking of (it could be anything - what I've got to do tomorrow, going over what I did today, etc etc) and resolve to concentrate on breathing, only breathing, nothing else.
To accomplish this, I breathe in and out in a natural and unforced rhythm. When I breath out, I think the the word 'one'. I don't actually say the word but only think it as I'm breathing out. I continue to breathe in and out and continue to think the word 'one' on the out breath. I try to concentrate on this only - to the exclusion of all other thought. If I find myself wandering away to my love life, my day, etc, etc., as soon as I am aware of it, I go back to breathing in and out with 'one' on the out breath. I keep this up. Sometimes it feels like forever, but sure enough, if I keep it up, I'll fall asleep. When I wake I'm surprised that that technique put me off to sleep so quickly - no more than about 20 out breaths and I'm gone!