This is a tip especially concerning whiplash pain, migraines and headaches. I learnt it from my amazing chiropractor Kyrre, who unfortunately passed away too soon.
If you have suffered even a very slight whiplash injury, more often than not the neck extensors become over-stretched and as a result your neck will become slightly unstable. To compensate for that you may unconsciously begin to tighten your jaw muscles in order to stabilize the injury. Going months and years with clenched jaws leads to all other kinds of problems, most notably tension headaches and neck pain. This is what in many cases exacerbates pain in whiplash injuries that are too small to be detected.
By working to unclench and relax the jaw muscles, and then gradually training the small support muscles in the neck that are supposed to hold the weight and stabilize your neck, you will remove a common source of pain, and relieve yourself of many headaches, quite literally!