Ive been having many of these issues myself. Here are some of the main reasons why my tires have been going flat:

- Unnoticed hole in tube.

Just fill the tube up with air and put it in water and try to find any holes, then just patch 'em up. Its easy as hell, and you'd be surprised how hard it is to see holes w/o water.

- Debris in the rim

Anything from glass to a spoke poking through the rim tape will cause you a flat. Like people mentioned before, just take it to your LBS and have them install new/better rim-tape.

- Too much inflation

Keep in mind the recommended PSI to inflate your tire doesnt include the weight of you on the bike, so keep it 10-20 psi lower than recommendation, but still enough to maintain a good speed.