You might not believe this tip, but I’ve read about this time and again. You should put your tongue on the roof of your mouth whenever you exercise, especially when doing something that might typically strain your neck.
Putting your tongue on the roof of your mouth helps to reduce neck strain and it has been shown to actually increase power and strength. The theory is that it helps you better align your head and your neck during exercise, which will help reduce the potential for injury from muscle imbalance and poor form.
Never criss-cross your fingers behind your head either. This usually causes people to pull on their head or neck when attempting to situp or crunch. Instead, simply place your fingertips on the side of your head or behind your ears, or keep your arms crossed over your chest. Try to pick a spot on the ceiling or high on the wall, and focus on that spot throughout the movement. Try to never look down or even straight ahead if possible.
Another tool you can use is a mouth piece made specifically for your mouth, that will help keep your teeth and jaw aligned during exercise. I have read on numerous occasions that this helps increase concentration and strength.
Sadly I don’t have any links or resources to back up this claim, it is just something I have heard several times over the years. Could probably find some research on this if you search Google.