I've done these things over the years when I have this trouble:

Mentally disassemble and reassemble, naming the parts as I go, an M-1 rifle (this dates from basic training). Talk about boring you to sleep!
But you can use any fairly complicated thing which in the past you knew very well but haven't used in years.

Create a science fiction story in my head, with emphasis on being very exact about how starship drives work, etc. VERY exact.

Try to remember all the words to a song I haven't sung in years.

Read. Not anything exciting, but something like a book of cartoons or even the instructions for the tax forms.

Do yoga -- breathing and relaxing as suggested above.

DON'T drink alcoholic beverages and DON'T depend upon pills (unless you really DO need them, preferably for something else).