akkando,

Here's what I do:

1. pull everything apart and run a cotton ball along the inside of the tire. You do carry cotton balls right (ha ha)? The cotton will pull on anything stuck in the tire. If something is stuck then time to get it out.

2. Run a finger along the inside beads and trench of your rims. Pull back a bloody digit? Rim issue - time to change and next time run your fingers over slower.

3. If 1 and 2 are good then you know for sure it's a tube issue. I pump some air in the tube and pull it under my nose until leaking air gives me a tickle. Kinda' fun in it's own way. Repair with some form of speed patch and get back to it.

Actually, I check 1 and 2 and throw in a fresh tube and deal with the puncture when I get back home. The reasons why a tube flats out are myriad but if you are flatting regularly and 1 and 2 are good then it's probably pressure.