Perform cardio activity before your workout. When the body is warmed up as a result of cardiovascular activity, you're able to get a better contraction on your abs when you work them. The warm up enables you to better feel the deep muscular burning sensation when your abs are trained. Contract and flex your abs in between sets. This technique helps to build abdominal definition.
Bodybuilders have been know to stretch and flex in between sets to increase muscle definition.
Minimize rest time in between sets. The goal is to keep rest to 30 - 40 seconds in between sets. Focus on the diet. Your diet is the main factor in how your abs will look. This is one of the most frequently asked questions I get.