1. Do all the usual sleep advice: no caffeine the day before, no electronics in bed, no bright lights, etc. Make sure the only thing keeping you awake is anxiety.
2. If anxiety is the only thing keeping you up, then the only way to get sleep is to reduce the anxiety. Different things will work for different people, it's worth experimenting to see what works best. There are a million different ways.
3. If you can't calm down, just accept that you may not get any sleep. For the vast majority of situations, a night without sleep will not hurt you or your performance as much as you think. Sometimes the best way to calm down is to realize that the stakes are not as high as you imagined and that you are going to be okay.

All the "sleep hygiene" advice helps, but I also find listening to a lowkey podcast in the dark distracts and relaxes me. When I start drifting off I stop the playback and (usually) fall asleep.

All the other comments are tips to get you to sleep, putting yet more pressure on trying to actually sleep, which then makes it even harder. Then all the stuff you put in your body tires you even more and all the worry exhausts you, making things worse.

Instead of needing to sleep, just tell yourself that it's fine if you don't sleep at all, you're in bed, you're resting, and if that's all you get to do for the entire night, you'll do fine and have had enough rest to get through the day.

The act of letting go of having to sleep puts you in a state where you will most likely fall asleep anyway, and if you don't, that's fine too.

Mantras.

As you’re trying to fall asleep, repeat in your mind ‘I will awake at seven alert and refreshed’. (e: or six or five, or whatever, but keep a rhythm.)

If your thoughts are intruding, say it aloud. Keep your phrase to four beats (awake, seven, alert, and refreshed should be even beats). Keep saying or thinking it, over and over, until you fall asleep. Sync it to your breath and heartbeat.

It sounds stupid and simple, but it works.

Melatonin

Paying attention to my breathing has sometimes worked (though not always) (not changing it, but feeling how your chest goes up and down and hearing the air rushing in and out and maybe even picturing the rhythm in your head). It distracts me from other thoughts and calms me down.

If that doesn't work, I try to distract myself with an interesting fiction book. That normally brings my head to less adrenaline-inducing thoughts (since now when trying to fall asleep I will be more focused on who the murderer of the dead child could be, or why the Zogophonts abandoned that resource-rich planet)

Cannabis, heavy indica. Some sort of Kush usually that I keep in reserve, I rarely smoke these days otherwise. Knocks me straight out in this sort of situation, great sleep.

Don't overdo it or you'll be all baked and goofy feeling, you just want to be relaxed and sleepy.