The strongest, straightest ground strokes come when you are shifting your weight toward the net as you contact the ball. A good transfer of weight begins with alert anticipation of your opponent's shot. If you aren't ready, you will be caught with your weight on your back foot.
Trying to hit the ball on the rise is a good way to shift your weight into the stroke. You'll also borrow power from your opponent and send it right back at him.
Another good idea is to think about leaning into the shot. Your weight will be moving toward the net rather than backward as you swing through the shot.
Shifting your weight into your ground strokes also can encourage you to rush to the net behind a solid hit, which will put your opponent back on his heels instead of leaving you back on yours.