you'd think that they'd do that now woudn't you -- the "issue" with the TPS is that it ahas a rotating shaft that's supposed to be sealed -- and the damned driver will be turning that shaft (throttling) while the sear area is exposed to some rather agressive vapors (steam from mud/creeks, etc).
Some brands seem to perform better longer, but I havent seen much that can be done. -- one of the most interesting was to place a thin felt washer (like found in a piano's action) over the TPS's shaft where it entered the sensor's body that felt was coated with non-aerosol WD40...
It seemed to keep things happy enough, but there really isn't much room there for any thickness and since the TPS moves one-way on it's own spring any binding is a bad thing.
I've seen (and taken to thismyself) folks add a little bit of silicon dielectric grease to the outside of the seal -- thinking is that the Silicone will act to repel water and the first thing past the seal will be the grease which should help the seal seal better... I've not had the chance to "test" that in any controlled way at all...