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)