Bike commuting: choose goals by increasing distance
A lot of people who want to start utility cycling pick "ride to work once a week" as their initial goal. There are a lot of reasons why I didn't start with that -- including the fact that my office is almost 20 miles away from my house. Instead I picked goals based on distance, and tried to ride to the destination EVERY TIME I needed to make that kind of trip.
I started with the supermarket and ATM machine one mile away from my house, where I shop several times a week. When I was comfortable with that trip, I started riding to the train station 2 miles from my house almost every day. Soon I challenged myself to get to the farmer's market 5 busy miles from my house every Sunday. The first time I did this ride, I was scared but exhilarated... I bought myself a pastry to celebrate my first double-digit ride! After that it was just a matter of time until I reached destinations that were 8, 12, and 20 miles away.
The key is that I kept adding distance AND frequency with this strategy because once I knew I could reach my target I tried to bike there every time. I think it's important to make cycling a routine groove, where you automatically reach for the bike instead of the car keys... which means it's better to take a short trip three times a week rather than a single longer trip once a week.