So stop bluffing!
If not, bluff on flop, bluff on turn and bluff a lot on river... I have exactly the same experience as you by the way, but I didnt know it went all the way up to 20 dollars, I thought the donkey calling stopped at 1/2$ but I might be wrong.
But ask yourself, are you bluffing so frequently that actually some of these donks actually got some kind of a read on you?
I mean if I hit the bottom pair I got no problem calling a minimum bet and start fishing for the second pair or maybe even trips. I wont go all in unless I have a very good read on the guy, have done that a time or two, and then I am usually ahead, and if he gives up on the betting on turn and a minmum bet on the river I usually makes the call, and when this happens I win in more of 60% of the hands