My Favorite Bike Commuting Tips
This page mostly has tips about cycling (actually on the bike). There are a ton of other great general tips about making bike commuting easier on the Overcoming Obstacles Page.
Try Clipless pedals
OK, in the biking world, "clipless" means that your shoes DO clip in to the pedals, like a little ski binding almost. You could call them "clip-in" or "cleated" too. I'm talking about shoes that click into your pedals. If you haven't ridden a bike in a while, save this tip for later, but eventually, you should give it a try. Most people get used to it after a few rides and the rewards are awesome: more power, more efficient, faster up hills.
Raise Your Seat
I've experienced this 100 times, and I'm still amazed every time. There are a lot of people riding around out there with their seat too low. Try to get high enough so that your leg is almost straight at the bottom of the pedal stroke. This makes pedaling significantly more efficient (easier), especially on hills. Eventually you might consider a professional bike fit at your local shop.
Pump up your tires
Allways keep your tires at or near the maximum recommended pressure as indicated on the side of the tire. If your tires are low on air the bike will be very difficult to pedal and you will get flats. They probably need more air than you think. The old squeeze test is not enough. Get a good floor-pump with a guage and inflate them to the proper pressure (80-120 Lbs for road bikes). More about tires here.
Keep your old innertubes
I've used old innertubes for quite a lot of things. Usually they can be patched in the comfort of your home and brought along on your next ride as a spare. However if you really destroy one, they can be cut up and used as "filler tape" under clamp-on lights, speedometers or fender mounts, etc. They are soft, grippy and besides keeping your accessories clamped on tight, they will protect your bike's precious paint job. This reminds me, check out these cool handmade bags, made from recycled innertubes and tires.
Always ride with a spare innertube
Don't mess with patch kits when you're on the way to work. Bring a spare tube, to fix your flat on the road, and patch that tube after you get home. Although if you get Gatorskins this isn't likely to come up very often.