Eat right and work out hard. The key to six packs is low body fat. Crunches, leg raises, leg lifts, planks etc will all hit your abs, but if you have too much fat, they won't show. You need to be burning off more calories than you are taking in.
I agree with SXI, the key is a clean diet to reduce your body fat %. You also have to work the abs sufficiently at least 2-3 times per week with a day of recovery in between ab workouts.
There are tons of ab exercises, some of which SXI listed above. If you need some more ideas, you can find a few here:
