I surprised this one hasn't been mentioned.
I use an exercise called floor cleaners to get that pump going in my abs. It actually involves a bench so I guess you could call it the "bench press" for abs.
1. Put your warm up weight for the bench press on the bar
2. Lie down on bench and un-rack the bar
3. Hold the bar over your head with arms fully extended
4. Do leg lifts while holding the bar up.
You'll only be able to do 15-20 to start but it's a great way to get the pump going. It's effective because instead of lying flat on the floor and doing crunches you'll be using the core muscles to balance your body on the bench along with the ones actually raising the legs.
It's ok to bend your knees a little when coming up to take stress off the hip flexors/saccral region.