Unfortunately, I've been a chronic insomniac since long before the head injury, so I can commiserate. I don't know about how things differ with insomnia caused by PCS vs. my long-term version, but a few things that help me, mostly with falling asleep (not staying asleep, sadly):

1. Good sleep hygiene. Try going to bed at the same time every night, including on the weekends. Keep electronic devices out of the bedroom entirely (tv, computer, even smartphone) - don't use them in bed, of course, but keeping them out of the room is still better. Don't nap during the day if you can avoid it, and don't drink caffeine after ~3pm (if you do at all).

2. Practice relaxation techniques - one that I've found helpful is this short little bedtime yoga routine (https://www.youtube.com/watch?v=CgcW_4vssvM). You can do it in your bed, and it's very calming.

3. A cup of chamomile tea or another hot beverage can be helpful for me a bit before bed. If you're willing/able to take a non-habit forming medication, 1 benadryl can also do the trick for getting to sleep more quickly, though I find that I typically wake up once it wears off on the occasions that I do use it.

Good luck!

ETA: a few dabs of lavender essential oil is another thing that a friend recently recommended. It didn't work for me, but it works for her so it's worth a shot!