Touch your heels

This exercise works on 2 areas: your six-pack and your oblique muscles, which give you a firm waist.     

Beginner
3 sets of 8 reps (both sides)
1 min recovery in between

Intermediate
4 sets of 13 reps (both sides)
45 sec recovery in between

Advanced
5 sets of 20 reps (both sides)
35 sec recovery in between

Very Advanced
6 sets of 30 reps (both sides)
30 sec recovery in between