You will need to strengthen the abdominals and the leg muscles, especially the hamstrings.

Abdominal exercises will help to pull the pelvis back into neutral position.

Contract your abdominals and tuck your buttocks under and you should notice your pelvis tilt backward. Hold this position for 10 seconds and then repeat.

Also, complete trunk curls or crunches. Make sure you are only using your abdominals, though. You should lie on the floor with your feet flat and knees bent. Place your fingertips behind your head for support. Contract your abdominals (pretend someone just punched you in the stomach) and slowly lift your upper back off the floor. You head should be the last to leave the floor. Slowly return to the floor keeping your abdominals contracted until you have complete 10 repetitions. If you do these slowly and with control, 10 is all you'll need to do.

Do Not Do SIT UPS! Sit ups require your hip flexors to contract and lift your upper body past the "trunk curl" position. This causes your pelvis to tilt forward, which is exactly what we're trying to correct.