1. Ride faster, see how close you can get to 30 minutes.
2. Add a few miles to your commuting.
3. Shave some time off of that.

That would be the short version. Basically you don't need to make a straight line for work in the morning; build several routes of various lengths and inclines. Look for variety and, push comes to shove, on some days take your bike on your car (provided you have one) and commute/train from a whole different direction with better traffic or hills.