I don't commute regularly, but I did commute when I was training for a cross-state tour earlier this year. My commute was also 18 miles, in order to take safer bike routes (only 11.5 by car...)

One way to do is half-time:

Day One: Drive your car, with your bike, to work, bringing the clothes you'll need to 1) ride home in, 2) wear at work the next day. Ride your bike home

Day Two: Ride your bike in to work, shower/change etc, then drive home with the bike.

Like I said, I don't do this on a regular basis unless I'm training for a sepcific event, but it works for me.

Another way that I will try the half-time next year is to get the bus in in the morning (they have a bike rack), and just ride home at each night...

Good luck, have fun, be energized!