You may be able to purchase new limbs directly from PSE in order to increase the draw weight. Try contacting PSE or ask someone at your local pro shop. Other than that there really isn't anything you can do other than increase your draw length (which won't do too much) to increase the poundage.

On another note, 40lbs. is enough to take a whitetail with as long as your shot placement is good, you're using cut-on-contact syle broadheads, and your shot placement is good. Many women and younger archers successfuly take animals every year with 40lb bows.