Constipated baby

What can I do to help her? I have her on soy formula. And breast milk at night or when I'm home. I can't switch formulas because I've tried so many and soy is the only one that she seems to handle. If I give her a milk based one, she screams after every feeding. But can I do anything to help her with the constipation? I hate watching her struggle to poop. :(