For background, I started commuting about 2-3 months ago. My commute is about 10 miles, mostly flat but a few small hills, a lot of narrow two-lane semi-country roads (it's a former rural area that is turning suburban). My bike is a Haro Express Sport with stock tires.
I'm a pretty slow 11 MPH average right now. Although my main philosophy behind bike commuting is that slow bike commuting is better than no bike commuting, I think I'd really like to increase my average MPH, in order to A) help increase my fitness, maybe get a little faster so that I can go riding with other people and not be the slowest one in the pack, and B) reduce my ride time so I can sleep in a little longer.
Do you have any suggestions for increasing my speed? I'd prefer not to buy another bike but am open to modifications on my current one.