A few questions: How old is she? How long has she been pitching? Does she have sound mechanics? Does she have accuracy down?

There are drills to help with speed, barring all the above are "yes". One drill is to have her throw as normal and then move her back a couple of steps after each throw until she has to bounce one into the catcher. This will get her to push off more with the back foot and drive into the ptich.

Another drill. Tie elastic surgical tubing (or something similar) around her waist and to the chain-linked fence. Have her move away from the fence until the tubing tightens a bit. Facing away from the fence, have her pitch normal. In doing so she will have to get a good push off as the tubing is holding her back.

These are two drills for pushing off the mound and can help with speed.

GOOD ANSWERS !!!!!!!!!!!! I'm going to STEAL THEM FROM YOU !!!!! lol

Scott is right with the questions he asked..How old and how long has she been pitching? If you have a young and new player, pitching mechanics will be the focus on helping her to increase her pitching speed. If you have an older pitcher, conditioning her physically will be a factor on improving on what she knows. Building core and leg strength and balance will help

Here are some drills
http://www.softballperformance.com/blog/softball-pitching-a-drill-to-increase-pitching-speed/