The best exercises I know for a lower stomach is leg lifts.
1. lay down (your stomach pointing up) and place your hands under your bottom or use a rolled up towel to help support your lower back. You should be comfortable. If your not adjust your hands or towel until comfortable (if you have lower back problems talk to a doctor before beginning this exercise)
2. SLOWLY lift your feet off of the ground about 6 inches to 18 inches while keeping your head touching the ground.
3. Hold for five seconds or as long as you can.
4. SLOWLY return your feet to the ground. Dropping your feet defeats the purpose of the exercise.
Begin with 2 sets of five leg lifts and adjust accordingly.
variation: while your feet are up in the air move your right foot above your left foot and visa versa. This helps with your oblique (sides of your stomach) muscles.