i had a similar dilemma till a roadie turned me on to 1 minute intervals. dead sprint as fast as i can go for a minute. if you're doing it right you'll feel like puking by the end of that minute. then 4+ minutes of rest, then one minute on again. a few of those a day or two a week will go a long way.
this isn't really applicable for the commute, but can be done before or after. it doesn't take much time and doesn't require a huge amount of space or a long path, just a relatively uninterrupted loop.
edit: oh yeah, joe friel's book "the cyclist's training bible" has a lot of good info too. kinda sucks the fun out of riding if you think about it too much though...