Tires keep getting low

Two weeks ago, my rear tube popped after going over a huge bump.  I replaced the tube and went on another ride.  Two days later, I was ready to go out for another ride, and both tires were severely deflated-(the old front one and the new back one).  I pumped them up, went for a ride, and the next morning they were near flat again.  I took the tubes out  to check for holes, but couldn't find any.  I'm using tubes with presta valves.  Could I be doing something wrong that is causing the valves to leak?  Over-tightening, perhaps?  I just don't want to buy a new tube every time I go for a ride.  Please help!