Think you can throw decent strikes? That's what I thought. Well, try that on the ground and the whole thing changed. After going through few sessions of ground and pound (GnP), this is what I learned:
1. Being pounded on from the top is terrifying - think he's the hammer and you're the nail, or Thor raining thunderstorms on you.
2. Striking him from the bottom is really tough, and exhausting.
3. It's very hard to defend strikes from the top, esp. from an open guard.
4. You need to be very fit to do ground and pound (from whichever position).
To defend from the bottom:
1. Keep a tight close guard.
Cover your head with both your hands.
3. Sit-up to left or right whenever he pound down and grab his arm, one at a time.
4. Grab down to his wrists.
5. Pull his hands under your knees as you change to open guard (or butterfly guard) by resting your feets on his hip. Kick his face from the hip.
6. Work your escape once you have controlled his arms this way.