I suspect one of three problems may be occurring here.

1. Far too often when replacing the bibb washer on a faucett stem we simply turn the water off at the source, unscrew the stem, replace the washer, then screw the stem back in place and turn the water on again. Sounds simple enough except we have omitted one vitally important step.

A faucett stem is a screw function device which means that as the bibb washer becomes worn the stem will screw in slightly deeper to compensate for the wear until the stem eihter bottoms out or the bibb washer is complete worn out. The problem is that when the stem is removed, the washer replaced then the stem is immediately screwed back in the new washer is compressed to the depth of the worn out washer thereby instantly destroying the new washer. The solution is easy. When we pull the stem and replace the washer we must remember to turn the stem to the full "Open" position before screwing the stem back into the faucett body and once the stem is tightly fitted in the faucett body we then turn the handle to the closed position. In this fashion the stem will only rotate inward until it makes the proper contact to turn the flow off without distorting the new washer.

2. While some generic or low end faucetts have permanent seats nearly all mid-range to high end faucetts and shower mixers have replacable seats. When the stem is out check the face of the seat for chips, burrs, sharp edges or mineral scale that could be abrasively effecting the new washers. When in doubt change the seat. When selecting a new seat here is a little tip. Faucett seats are generally available in three grades, Brass, Monel-Metal and Stainless steel. While the stainless steel variety may cost 3 or 4 times as much as the brass seats in regions of hard or acidic water the brass type may last 2 to 3 years while the SS will last nearly a lifetime. When installing a new seat apply two or three wraps of the white single density PTFE (teflon) tape to the threads before installing the seat.

3. The third possibility is not a problem with the faucett but rather a common user error. Often when people turn a shower mixer off they desire the water flow to stop instantly but it must be remembered that there is some residual water in the line from the mixer to the tub spout or perhaps still inside the shower head that will continue to drip for a few moments. When turning the faucetts off turn the handle until it feels snug then wait a few moments for the residual water to clear then apply a bit more tension if necessary. Far too often the seats are destroyed by someone being a bit heavy handed.

Wow, okay, after my dad tried to fix my leaky shower faucet FIVE times, I found this response and am now very hopeful that we found and fixed the problem (we addressed #1 and 2 above!). Fingers crossed, amateur plumbing tools put away, hoping for a full recovery! Thanks so much for the detailed post!!! What luck to have stumbled across it in my google search for "repeatedly leaky shower!"