First, go to a low iron formula.
Too much iron causes constipation.
Then try giving her 1-2oz of diluted fruit juice, such as apple-prune or prune juice.
You can also help her by "bicycling" her little legs and letting her push her legs against your hands, or laying her across your knees with her tummy on your leg and rocking your legs back and forth while firmly patting her diaper. Those exercises tend to help move things along.
Best of luck!
ETA: Yes, you can give fruit juice. Just stick with the same kind and be sure to dilute it with water. You could also add it to the formula.
Beth is also right...Karo syrup also works. My own mom used it on me as a baby. I preferred to use fruit juice with my boys as it's not straight processed corn sugar but natural fruit juice. But either will work.
ETA again: In response to above: Diluted fruit juice as a treatment for constipation is NOT recommended for babies less than 3 months old.
If your baby is aged 3 - 6 months, offer 1 oz of diluted juice (2 oz if he's over 6 months). Offer this twice a day until his poop is soft. Give less rather than more to start with as too much juice can result in abdominal gas, bloating and diarrhea. (See Carbohydrate malabsorption for more information.)
http://www.babycareadvice.com/babycare/general_help/artic...
I do research this stuff. ;-)