Everclear is the best way. Its 190 proof/95% alcohol and will get every last bit of the old taste out. Just fill up a shot glass and soak them in the alcohol for anywhere from 10 minutes to a few hours. Whatever it takes to get the old
juice out.
I run water through my attys first in order to keep the alcohol rather clean and untainted with juice so I can reuse it.
If you can't get 190 Proof Everclear look for the 151 proof and if you can't get that use 100 proof Vodka.
Make sure you thouroughly dry them afterward. A lot of people mistake a dead atty for a flooded atty. Canned air works great, followed by a hair dryer or just sitting them cart side down overnight.
I've never ruined an atty this way. Attys are meant to get wet... Dry burns on the other hand can put some strain on an atty but usually won't ruin it.