You can do Abs everyday, but if you train them the right way, you should only be doing them once or twice per week. If you train each muscle group once per week, then I'd treat abs in the same manner. Twice per week--same deal. The way you can accomplish this(and this leads me to your second question), is indeed by using resistance when training abs. I still see people doing a zillion reps on that ab crunch device. This accomplishes nothing. In my opinion, that is the same as performing 100 reps for Chest with just the straightbar with no plates added, and calling it workout. It is rediculous if you think of it that way, eh?! For abs, I would stick with exercises where you can add weight, which will allow for 7-15 reps, depending on what you want to work with. A few of these exercises are:
1. Machine Ab Crunches.
2. Hanging Leg Raises-legs straight.
3. Straight-Legged Reverse Crunches. Performed on an incline.
4. Rope Crunches.
5. Inclined Sit ups.
If the exercise listed cannot done on a plate-loaded machine, then you can easily keep the targetted amount of reps in check by doing a pause & squeeze at the top of the rep, or simply by slowing the rate of motion of the rep, down.
You can finish up with some crunches if you want, just to burnout. 30 reps on those at the end of an ab routine is fine. I usually try to do 2 sets of each of the lifts I listed above. Done this way, my Abs are as sore as my Chest would be, after a heavy day of benching, or Back after a heavy deadlift day. Given that, I see no reason to do abs for another 7 days--same as any other muscle.
Sidebar--I feel the same way about calves as well. Done correctly, there are no muscle groups that "need to be trained" or "can be trained" every day or eod, or whatever.
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