How long are you pumping? Increase your pumping sessions by 5-10 even 15 minutes. Milk production is a supply and demand thing; make your body 'think' it needs to make more. You might not be pumping anything during those extra minutes but you are telling your body "hey, make more milk will ya?".
Keep hydrated. Water in does not equal milk out, but the process of milk production is a chemical reaction in your body that requires water. Keep yourself hydrated so that the 'milk making' efforts can happen efficiently.
Eat oatmeal. I can't explain this one, but it worked for me while I was exclusively pumping (8xday for 15-25 minute intervals...).