How do I increase my breast milk supply?

I had the exact same problem with both of mine. My doctor said that I wasn't eating enough calories, but no matter how much good fat, calories, etc I "dried up".
One trick that worked with my first son temporarily (only try AFTER you talk to a doctor - my doctor actually suggested this to me)...
Pump as much milk as you can (or have some formula ready for a few hours). Drink 1/2 to 1 can of beer. After a couple hours (you will feel "full") pump like crazy! Get it all out (discard) and wait another 2 hours (to fill up again) and pump again.... That will increase your milk supply, but make sure you pump and discard any milk that may have alcohol in it! For some reason beer will make your body produce more milk temporarily. This is a quick fix though until you can figure out a better way.
Remember though, some women just can't breastfeed. Our bodies aren't as able. I only fed my 1st son 3 months and made it less then 4 WEEKS with my 2nd. No matter what I did it wasn't happening. So don't feel bad if you can't. At least you tried! You will soon notice the benifits of not breastfeeding as well - i.e. you boobs don't hurt, easier for other people to help feed, and I personally noticed formula babies sleep better at night!