If you have replaced the washer then obviously the problem lies elsewhere. My guess is that the valve seat-found inside the faucet valves, against which the washer lies-is the culprit. Over time, the valve seat can develop small ridges. Water is thus not being sealed inside the valve and that is what is causing the water to drip. The way to solve the problem is to buy a dresser, a small grinder that can be inserted into the valve. The tip of the dresser, when turned like a screwdriver, will smooth the surface of the valve seat and remove any ridges. Once it has been smoothed, a new washer will complete the job. If the valve seat is actually cracked, then it will have to be replaced. Buy an inexpensive valve seat wrench, valve seat, and washer for this job at the hardware or plumbing store.