(easy) For a more effective workout, should I...?

Here's the deal.
When I workout, especially while doing ab exercises, I'll feel like exercise while I'm doing it, but afterwards (an hour, five hours, a day) I don't feel any kind of soreness. Should I go for less reps, but more sets, or stick to going continuously until I can't any longer? Usually if I do that, though, I start feeling the exercise in my lower back. What should I do to actually feel the burn?
At least a halfway educated answer, please.

Ok, soreness is a poor way of determining how effective a workout was, so don't use soreness as a test. It's good that you feel your abs working while doing the exercise, you probably have good form.

Less reps and more sets and weight will help you add more muscle mass to your six pack. If you start feeling the exercise in the lower back it is due to fairly weak abdominals. As you strengthen your abdominals your lower back will no longer be used to support you during the exercise.