Put your beds cover down, folded over if it's a double, s'what I do

Even using your bed can help. Just do crunches, or make sure you don't use the bed to spring back and cheat. Or you could get a yoga mat.

Do crunches; they're better than sit-ups and since you don't actually sit your tailbone shouldn't move at all.
www.youtube.com/watch?v=1V4RXxLHNCY

I've always heard that crunches work just your upper abs, while sit ups work the whole area. In what way are crunches better?

as soon as your lower back*

Because as soon as your back leaves the floor you're only using your hip flexors [particularly the psoas major] instead of your abdominal muscles, so the rest of the movement is pretty much wasted and actually could result detrimental to some people's posture in the long run (hip flexors already overpower hip extensors [gluteus maximus and hamstrings] on most people who spend a decent amount of time sitting, aka almost everyone)

makes sense, thanks man