Hi J.,
You have gotten some wonderful answers and I just wanted to add my two cents worth.
First I would suggest that you think about why you feel your milk supply has diminished? Is it because your breast feel softer? That could be normal adjustment post engorgement.
Is it because your baby seems fussier? That could be that she is getting older and looking for more stimulation.
Is it because she nurses for shorter periods of time? She may just be getting to be a more efficient feeder. Or, is it a weight gain issue? That could be just a normal weight gain pattern for her. Is it because you are able to pump less than you were before? That could be equipment malfunction.
If you decide that you truly do have a supply issue then I would suggest that you add pumping to breastfeeding if you haven't already. About one hour after you feed your daughter at the breast double pump with a good electric pump until about 2 minutes after you last see milk being expressed. Also, some folks have suggested more water which really won't do anything unless you are truly dehydrated. However, if you are not taking in enough calories your milk supply will decrease. Make sure you are getting about 500 more kcal per day than you normally need to maintain your weight. Do not try to diet.
If your baby doesn't seem to be gaining enough you could feed her the milk that you pump rather than saving it and/or you could try breast compression to increase the amount she gets at each feed. Go here for a handout and videos on breast compression: http://www.breastfeedingonline.com/newman.shtml
Also, think about anything you might be ingesting like pseudoephedrine, hormonal birth control, ergotamine, bromocriptine, carbergoline or alcoholic beverages. These things can interfere with milk supply.
I find myself writing such long answers because it is easier to personalize my advice when I have more give and take with the Mom. So, I suggest that you contact a lactation consultant near you for more individual assistance. I had a Mom come to me recently complaining of a decreased milk supply. We weighed her infant on a hospital scale and then Mom fed her for her usual length of time. The baby nursed for about 15 minutes total and when we weighed her afterward we found that she had taken in over 6 ounces!! You can go to flca.org for a list of LCs in your area.
K. Ianacone, RN,MA,MSN,CCE,IBCLC