I do old school military style situps- Lying fully on my back with knees bent with both feet down, the I sit up while my hands/arms pass by my knees, about where elbows reach knees(feet stilll on ground), and back down. I do them typically in family room (carpet has a good premium pad) while watching TV. Or in hotel gym if I'm traveling for work.
I find that at about every 50, my body works its way to back of room and I have I have to crab walk back forward to front of room. Its also a mental challenge to keep count correct as I get lost in routines.
I've never had back issues, I've always gradually increased counts when I would get back into routine after breaking for a number of years: first to nightly 20 count, do it for a few weeks. Then one more each night, then I'm feeling stronger, increasing up it to 5 more more.. similarly increasing 20 more, etc.till hit nightly 1000. Then I push myself with times to reach nightly 1000, using stop watch.
I've read so much how situps are bad for your back, do crunches instead, but I've never had back issues. Only issue is that if I use a hotel gym where mat is too thick and mushy, like over an inch, then I get skin rub on my back. In fact i've developed a skin callous on my right butt cheek(I guess I tend to favor slight right tilt when I do situps). I go throw a sweatshirt every month... I wear a hole in my sweat shirt's back side from the rubbing, then switch back to front till I wear a hole on that side
I'm experimenting/applying that technique to see if I could incorporate to achieve 1000 number on pushups(I've never attempted with sit ups previously.
I've learned, for me personally, its satisfying to maintain personal discipline and focus and learn and see what my body can do.
Also, I've applied a lot from the smart nutrition stories in t-nation, and elsewhere, and started to eat wisely.. hi protein(grilled chicken or grilled salmon), good carbs vs bad carbs: lot of veggies and fruit (staying away from grease cooked foods and processed or sugar loaded foods)