Sit-ups hanging from a jungle jim bar!!! Hold hands on head like you're being arrested and touch each elbow to opposite knee! That or old a 5-20lb weight on your chest!

You can also use that angled bench made specifically for that...

Sry I don't know the proper names but it works! Especially when you do them at a semi steady pace, stretch (like an inverted bridge or over one of those big rubber balls), then run for 5min straight, and repeat until you fall out....