Hi, You've already gotten lots of great advice. I've had to exclusively pump, and it wasn't easy going at first so I've gone through a lot of trial and error with it. Here's my two cents worth, some of the previous suggestions bear repeating;
1) Pump, pump, pump; stimulation is really the best way to increase supply. Nighttime is especially good as the prolactin levels are higher during this time.
2) Lots of water, lots of rest
3) Be comfortable when pumping, obtain a hands free device such as a bustier (medela makes one, but there are others too) to hold the flanges/horns and give you the ability to do other things that are relaxing, reading a book etc
4) Look at and think about your baby, pictures of your baby and/or smell clothes he/she has worn to increase stimulation.
5) Use the proper suction level on your pump(higher isn't necessarily better), use the correct size horns and dual pump if possible.
6) In regards to horns/flanges I highly recommend those made by www.pumpinpal.com They are cones rather than most of the other styles that have a sharp edge to them. Changing to these flanges made a HUGE difference in my comfort and therefore my supply.
Good luck with everything!
-Jenn