It’s all about routine. Here’s an example of how I manage it:
I get up at the same time every day, and go to bed at the same time too, regardless whether I have a day off or not, I never have a lie in. This way my body understands that time is my bedtime.
I only lie down if I am intending to sleep (same principle as above) and, unlike you may hear others say, I don’t read, watch TV, listen to music etc. in bed. The idea is that if I really can’t sleep I would get out of bed to do these things, again so my body learns what ‘bed’ is for.
I don’t drink caffeine or eat sugar after 7 pm, for obvious reasons.
It’s also important that you are comfortable in bed. Sounds stupid, but if you’ve been sleeping with two pillows for years and actually you’re more comfortable with one for combination sleepers… which happened to me, exactly.
It’s all very simple really. The difficulty is sticking with it.