-Before working on building speed, make sure she is throwing with correct form. Attend a clinic, call a local high school or college coach for help and make sure she has good form. Looking at your user picture, it looks like the person shown is not driving off the rubber and is bending over at release. She will get more power if she learns to drive with her legs and stay tall at release.

Once you are confident in her form:
-Use step back drills starting at warm up distance and taking a step back when she hits her mark 3 times in a row, throws 3 strikes before throwing 4 balls or some other goal. Keep stepping back until she can't throw with good form.
-Practice with weighted balls--only after well warmed up.
-Try tying elastic cord to a belt on her waist and driving with it anchored from behind.

Try this link for a few ideas on building speed:
www.pitchsoftball.com/default.asp

there is a thing i do called a wrist snap. you turn your hips toword the catcher and hold your wrist behind your hip and release at your hip. if you want your daugter to b come really good i sugest pitching lessons.

yes make sure she has correct machanics i am a pitcher myself and at 13 can throw 59 but she needs to use her fingers to snap the ball and her legs to push off the mound so i suggest build up her finger muscles by using like a 3 pound ball like a work out ball not like a actuall softball which is what i use also build her leg muscles to get stronger. some good books to buy for pitching are chery khemp she is a pretty good machanics pither her books are very good the publishing compony is human kenetics. but legs and finger/wrist snap are the best way to gain speed