I'm nursing twins, and for me the biggest things in increasing and maintaining supply have been, in no particular order:
Flax seed oil - both in capsules and toasted seeds sprinkled on foods.
WATER - like a gallon a day. Seriously. I know some say don't force the water, only drink to thirst, but I have to force it, or I won't drink at all and my supply drops.
Cal-Mag (3000 calcium/1500 magnesium) when my fertility returned. It seems to minimize the drop in supply around my period each month.
Besides foods, I have also found simply making sure I am getting enough food and rest is crucial. Not overdoing in anything is important, or my body shuts off the faucet.