I struggled for years to do sit ups without someone holding my feet. I have to admit I'm still not brilliant at them, but he are some tips for doing them well:
1) Try your hardest to maximise tension in your abdominal region when doing sit ups. I.e. No lying down or sitting up entirely during the exercise.
2) Imagine holding an apple between your chest and chin to prevent neck stiffness.
3) Don't focus on speed, but the quality of each repetition. For results, try to really focus on feeling the burn more than you have to.