There comes a time in everyone's life when you must decide: exercise or start saving for coronary bypass surgery. I've been running more, but sometimes things are just too busy for me to fit a run in. That, plus my previously-voiced frustration over the lack of progress in car fuel efficiency by automakers (despite existing modifications that can yield 100 miles/gallon), led me to think seriously about bicycling commuting.
I've ridden to work before, and do so when the weather is nice: one way. After work, my wife would then pick me and the bike up. The main reason for this is that we live at the top of one of the (if not the) tallest hills in the area. I decided that I would see if I could ride up, so that I could commute both ways by bike, and in fact it seemed feasible when I tried. I'm in OK shape, but from running, not biking, and only had to walk two half blocks (the steepest parts of the route -- steep enough that you might wonder if your car can make it). This leads me to believe that I'll be able to do the climb without stopping one day, and it's always nice to have a goal to shoot for.
Next comes the strategy to avoid procrastination and backsliding ("I'm a bit tired today; I'll bike in tomorrow," where tomorrows eventually become months). Part of this involves buying equipment to set my bike up properly. I've got a 7 or 8 year old Raleigh R300 touring road bike; I outfitted it with fenders, added a flashing tail light, and am in the process of getting a good, bright headlight (I already had a rear rack and panniers). I also bought tools (I used to do all my bike maintenance, but got rid of all my tools after grad school). Now if I don't commute, I'll feel like I wasted all that money. I also estimate that we'll be saving about $20/week on gas (the commute is a bit over 5 miles, one way, we only own one car -- an all-wheel-drive Dodge Grand Caravan -- so my wife often drops me off at work in the morning and makes another trip in the evening to pick me up; it adds up). I get about an hour or so of exercise per day, broken into two parts each of which increases my metabolism for an extended period of time afterwards.
I found an enormous number of bicycle commuting resources on the web. I'll just point out one: Paul Dorn's Bike Commuting Tips.
P.S. If you're biking home from work and it's uphill all the way, eat a snack beforehand.