I'd like to add here that "lower abs" don't actually exist--your abs are two verticle muscles. So any ab exersize you do works the lower as well as upper part of the muscle.
Also, to get a "six pack" you really should work your abs as you do any other muscle, meaning don't aim for 200 crunches or leg lifts a day, and weights should be used in order to build the muscle (it is not true that you get a six pack by overworking and thereby shrinking your abs).
Abs should be worked once or twice a week. I do two exersizes. One is just a normal crunch on the floor, but I put a 45 lb plate on my head--four sets of 15. The other is a cable crunch at about 100 lbs, 4 sets of 15. I vary this every once in a while but the basic principles remain the same. I've been doing this workout for about 6 years & definitely have the six pack.