When working with DDs lower half, often times I have her go through the motion without a ball, having her focus on what we are working on. I think Carly suggested that. Often times we will do 5 without a ball, then 1 with..... 4 without 1 with... 3-1, 2-1, 1-1. We have had good luck making lower body adjustments that way.
Also Hal has a good thread on getting your hips out over your pivot foot to make sure she is in a good forward lean position before she pushes off. Hal's thread:
How old, athletic and motivated? I think many kids can add a few inches by using the hands, especially the glove hand, more explosively. Dry drives (no ball) with a target spot on the floor about 6 inches farther can help. I believe Carly recommends core exercises and plyo's for pitchers. Stairs, chairs and boxes are great training aids for motivated pitchers.
All good suggestions. I would also recommend making sure the push foot is pointing forward during the push off. And if you're working indoors try to get onto a field (or any place with a rubber sunk into dirt) where the pitcher can dig herself a little hole in front of the rubber, get some tilt in her foot, and really use the rubber like a starting block in a sprinting race. Some pitchers just can't get a good feel for pushing forward when the ground under their feet is flat.
If you're talking about girls who are 12-13 or older, you can also get them with a trainer who will recommend exercises to increase leg drive.
Find a hill with about a 10-20 degree slope. Have her pitch down hill. You can also use a power-drive device, either accomplish the same thing. Pitching down a hill will teach her to use gravity to initiate the drive, the body will need to fall forward with the hips moving over the balls of the feet before the initial drive.
Try to teach that the explosiveness of the pitch is the drive off the rubber, everything after that is flow and whip. So many pitchers try to save the explosion for their arm moving downward, which just shortens up the arm and makes them muscle the pitch.
Bounding can help too. Keep the head on level plane so the head isn't going up and down (which means she is skipping). You can add it into your warm ups. Good for timing too ihmo...so be sure to sync the elbows as shown.
Also be sure she is driving out, not up. It may be very little but every bit she goes up can take away from her lateral drive. Make sure she is not squatting, if her rear is down like she is sitting then loss of power. The things everyone else has mentioned. So many little things that can help.
Regardless if you like the Power Drive or not Coach White does a good job describing in the video.