My back kills when I try to do sit ups?

I agree. Situps are inefficient anyway so try something else. I've seen studies that say crunches on a stability ball worked the abs the most of various exercises tested. Though you can also just work the abs the same time you do other muscle groups with things like squats, lunges, deadlifts, pushups, pullups, etc.

Situps and crunches will hurt, if done incorrectly. They have to be done with a ROUNDED back. Abs action only. For both: Keep your finger tips behind your ears, look up to the ceiling. Exhale on the way up, round the back, contract your abs, suck in belly button. Roll slowly back to the ground, inhale....repeat.... You can tuck your toes under the couch to make it a bit easier in the beginning.
Crunches: shoulders off the ground. Situps: all the way up.
Double up on yoga mats, if you have to, but don't do them on a soft mattress....

Check out their exercise library with videos:

http://www.bodybuilding.com/exercises/finder/lookup/filter/muscle/id/13/muscle/abdominals

Train safe!

I agree there can be other exercises done that may not hurt your back and are just as effective. However, I enjoy doing them so I use one of two things, At the gym I use an abmat or at home a I use a small pillow. It goes right in the curve of your back to help support you

Not sure if this link works, but it's one of the better show-and-tells about situps that I have seen lately.

http://m.youtube.com/watch?v=jDwoBqPH0jk

Have a look at the slightly rounded back and the abs action.

There is always a suggested biomechanically correct way to do exercises. HOWEVER, every individual is different. And so are the size and proportions of our joints and bones. Sometimes, a slight modification in form of an exercise can be done, to assure that there won't be any injuries in the process. E.g. if the geometry of my otherwise perfectly healthy hip bone does not allow for a side split, then it's not an exercise that I can safely practice. Period.
The same goes for a crunch and a situp. The exercises have a purpose. If an individual has problems, not related to form, but to physical limitations, including but not limited to illness and injury, then DO NOT DO THEM! No determination in the world will make this a safe exercise! Not with a pillow or a mattress under the back. And please do not give advise to others on how to 'cheat' your way into doing them anyway! If in doubt, please ask a physical therapist for advise on how to safely perform your exercises!

Think about the health insurance co-pays......and train safe!!!!

You could substitute the "bicycle" and "scissor kicks" exercises instead. They are great exercises for the abs.

The bicycle and scissor kicks will still require a proper position of the spine on the ground, and as I have mentioned before, a rounded back, in order to activate the abs properly.
A much easier exercise is leg raises on parallel bars. Those can be done with different angles, intensity, height...with comfortably padded back and arm rests, and without pain on the spine.
If you want to go really crazy, try gorilla chinups.....that'll give you some abs....

So stop doing sit ups. They are useless when it comes to ab exercises.

Sit ups are not useless, but they do have to be done correctly and if you don't have the ab strength you could easily be engaging the wrong muscles and hurt yourself. Someone mentioned tucking toes under something but my trainer actually told me that is a bad idea because you will be likely to engage your hip flexors and I guess more chances for injury that way?

If situps hurt, try doing some planks! Those are amazing for core and there are lots of variations. You don't have to do situps!

Sit ups are not useless, but they do have to be done correctly and if you don't have the ab strength you could easily be engaging the wrong muscles and hurt yourself. Someone mentioned tucking toes under something but my trainer actually told me that is a bad idea because you will be likely to engage your hip flexors and I guess more chances for injury that way?

If situps hurt, try doing some planks! Those are amazing for core and there are lots of variations. You don't have to do situps!

Sorry, but as an ab exercise, yes they are. Sit-ups are inefficient and if they do anything, they fire your hip-flexors, not the abdominals.

If it hurts- don't do it.

If it's not a good exercise AND It hurts- DEFINITELY don't do it.

Outside of training for a PT test- sit ups are useless.

This. Do planks instead. You'll feel it, believe me.

There aren't many points in life where a crunch/sit up is normal movement pattern. The role of the abs and 'core' is more anti-flexion,anti-rotational so things like planks, ab roll outs and bicycle kicks, where the abs are flexed in one position to stabilise the trunk whilst the exercise occurs, are better than exercises that engage it in the way sit ups do.

Why are you performing sit ups, what are your goals?

So what you're saying is that when you're lying down on your back and you go to get up that you're not doing a "crunch/sit up" motion to get to a sitting position? I'd love to see how you sit up without doing that.