In the past couple years, I've only gotten maybe 7 or 8 flats. Most of them, thankfully, were not on my commutes. However, 2 of them occurred in the past 2 weeks on my commute home.

The first was my own stupidity - anxious to install and use my new rear wheel, I forgot to put the rim tape on. The fact that I had brought a spare tube didn't help much, since I forgot to bring the pump with me that day. Surprisingly I rode about 80 miles before any problems occurred.

The flat I got today was because of a heavy-duty staple in the road only about 2 miles from work. When I went to change the tube, I found that my "spare" was the one that had burst because of the lack of rim tape, which of course hadn't been patched. I'm glad I brought my patch kit with me.

Lessons learned:
1. Take your time, and install things the right way.
2. Don't leave your method of inflation at home.
3. Make sure your spare tube actually holds air.
4. Always carry a patch kit.

Each of them is basically a no-brainer, but I guess I've been a bit careless lately.