I tend to do 3 sets of 8-10 reps of weighted (I use a 20kg plate) crunches, unweighted hanging leg raises, which both hit the rectus abdominis muscle and weighted side crunches (both left and right) to hit the obliques.
If you're unsure of how to do these exercises, here's a site with instructions and movies (it also has abdominal exercises I haven't mentioned):
http://www.exrx.net/Lists/ExList/WaistWt.html#anchor1944764.
Initially do these exercises unweighted and once you start feeling more confident with the technique I would recommend increasing the weight slowly. Like any muscle group, you can overtrain by doing too many exercises, so stick to about 3-4 exercises (making sure you cover the obliques) and also don't exercise them on consecutive days, as the grow when you rest...I like to leave 48 hours minimum before working them again.
Also I wouldn't concentrate on just abdominal exercises if I were you, as you're more likely to bulk them up quicker by doing a whole body workout with weights. This is because the muscles when exercised release growth hormone, which leads to muscle growth and so by exercising, for example, the leg muscles, which are big, a lot of GH is released.
Diet is key in bulking too, as if your diet doesn't have the necessary calories (greater than what your body normally requires) and amount of protein, carbohydrate and fat, you're not gonna add any significant muscle.
Once the bulking is out of the way and if you so wish, you can make the abs more visible by removing the layer of fat that's covering it. This is achieved by reducing calorie intake back to maintenance and doing cardio work, such as swimming and running.
Phew...hope that helped.