Here a twist to the drudgery of working out the abs(core). Mix in a little masturbation fun. This is for men, but could be modified for women.
I tried this morning. Although, I got to the point of no return and then backed off. If I were to cum, I would edge until it was extremely hard and sensitive and then while working my abs let go while in a full crunch.
1. Lie down on the floor. (bed is too soft)
2. Start stroking your cock with right or left hand (your preference).
3. While stroking, slowing lift your shoulder off the floor and perform a crunch while stroking.
3. If you want to up difficulty, lift your legs off the floor slightly to and continue stroking.
Enjoy your cock and work your abs at the same time.
For variation, check out the various different types of ab exercises and think how you could incorporate some stroking time too.
Here is a link for some ideas: http://www.ab-core-and-stomach-exercises.com/index.html
Now you can really enjoy those crunches.