How can my wife increase her milk production? We are supplementing with formula, but she wants to breastfeed exclusively.

Breastfeeding. The best thing a mother can do to increase milk production is to put the bay to suck more. Otherwise, increase the amount of fluid intake. Some even suggest o drink "malta". Exclusive breastfeeding for 4-6 months is appropriate if something else is added bay won't suck enough. You may consider consult la leche league or nurses specialized in breastfeeding.
Maybe.. Good nutrition and proper rest, if possible may help. Fenugreek has been shown to have some positive effect. And in some women, studies have demonstrated that a short course of reglan, (metoclopramide) a prescription drug , is effective.Consult with her doctor.