How to prevent neck and back pain during sit ups?

Apparently its the positioning of your head that causes neck strain. Initially i placed a tennis ball under my chin to stop this but over time didnt need it. I focus on pulling myself up with my abs and make sure im not tensing my neck/legs. Doing the situps slower helps you get the feel of this, remember quality is better than quantity. When you have the hang of this and feel like you can do more and maybe faster, go for it. Do the tennis ball trick and note a spot on the wall your eyes meet, then remove the tennis ball and focus on that spot when you do situps (try not to pull your head forward) sorry if this answer is bunched/has typos, its 1am and im on my iPhone :p