Although not cheap, I have found the best way to permanently repair stripped threads is with the use of a Timesert.
++ TIME-SERT Thread Repair Kits ++ thread repair kits for stripped threads damaged thread repair kits stripping threads head bolt stripped, engine block stripped how to repair stripped threads.
When you get it fixed, see if you can find a 2-piece drain plug. You screw the large one in, and a smaller one fits in the large one. You pull the small one everytime to drain, leaving the large one in. it will never strip again...