Pepper with Tips Only
Start off the players with tipping instead of actually spiking the ball. This way they will start with some control in the pepper. In my experience, most girls cannot control the ball because the spike is usually out of control or one player is consistantly hitting the ball too hard. Have them start off with easy tips then work to aggresive tips. They should always be sure that they control the tip and allow the other player to dog the ball.
This is how I warm-up my liberos before volleyball matches. Have the players stand very close to each other and pepper as fast as they can. By this I mean quick set, quick hit, and quick dig. If done correctly and under control, this warm up gets them much better at peppering and also gets them ready for the game. Have players do this in practice to help with their reaction time.
Far Away Pepper
This is by far the hardest version of peppering. Have each partner stand on opposite sidelines facing each other. Have them pepper. See if they can long pass, long set, and long spike. This will really help them in the control of their hitting. This is obviously very important.
Things to Look For
When peppering there are few things that you should be concious of. First of all, always be in athletic position. A player should never be standing straight up. Also, stay on your toes. Another important thing is for the players to constantly be moving. Once they stop moving, they will get stuck and will not be able to play a ball correctly.