Don't ride hard every day because you'll stay slow and be tired. Ride intervals - 10 minutes as hard as you can sustain, 5 easy, repeat 3-4X is nice to increase sustainable power and not too psychologically taxing. Slowly add distance or intensity. Take time to recover - an easy week out of 4, an easy month out of 4. Exploit the opportunities you have on your commute - making a traffic light is a good time for a 15-20 second sprint.

There are good books and articles on this, like _The Time-Crunched Cyclist: Fit, Fast, and Powerful in 6 Hours a Week_. While you might not go with one of the canned plans, the principles are useful.