PC clamps:

squeeze and release your PC muscle. Start with a set of 20-30, and work your way up to sets of 100-200. Do at least 300 EVERYDAY, for the rest of your life. This will give you an erection harder than when you were a teenager. Continual practice will ensure you a rock hard erection, any time you desire, as well as giving you the ability to actually hold back your orgasm as long as you desire simply by flexing your PC muscle.