I've been using a spreadsheet to keep track of my flats for about 8 years now. Before I started tracking my working theories were:
1) Most flats happen on the rear tire
2) Most flats happen when the roads are wet
3) Most flats happen on tires that I've used for over 2000 miles
4) Most flats happen when I've been riding on roads with bike lanes
The rear tire theory has proven true (36 of 42 flats have been in the rear, and three of the others were tube or tire failures, not puctures).
The bike lane theory also seems to be correct. About a third of my flats have been slow leaks that I found either in the morning or at the end of the day without having had any idea when the puncture occurred. Commuting typically accounts for about two-thirds of my mileage and there is no way to avoid bike lanes entirely on my commute. The vast of my flats that weren't slow leaks have happened on a road with a bike lane. I've only had a couple of flats on recreational rides, even though I use less puncture resistant tires for that purpose.
The wet road theory has me kind of puzzled. It's demonstrably true that objects will penetrate wet rubber easier than dry rubber, but only about a third of my flats have occurred on wet roads. Looking at the details of my log, I'm tempted to form theories to explain this away, but the data seems to be telling me that I'm just as likely to get a flat when the roads are dry.
The mileage theory is harder to evaluate. I've gotten 18 flats on tires with fewer than 1000 miles, 14 flats on tires between 1000 and 2000 miles, and 11 flats on tires with over 2000 miles. On the other hand, the flats clearly start to group together when the mileage gets high. I have more total accumulated mileage at the lower numbers (I have a lot of bikes, so a lot of tires that haven't hit 1000 miles). Once flats start to seem frequent (about three flats within 500 miles) I tend to give up on a tire.