Understandably a frustrating problem boosted, let me give you a couple of suggestions, I am by no means an expert, but I hope I can help.

First of all, make sure the glow plugs you are using will work well with your engine in the first place. Have you tried a different plug to see if you ran into the same results?

Secondly, an engine running to rich can cause just as many glow plug problems as an engine running to lean. In my opinion(I have been running r/c car engines for the last 3 years, so forgive me if there are subtle differences with plane 2 strokes), running to rich is worse then running to lean on a glow plug. I am not saying to run your engine too lean, just that generally I experienced more engine related problems then glow plug problems when running too lean. Try to tune the engine with a tach if you can to make sure you are tuning the engine optimally, and use a temp gun if you can to measure the temp at the plug. If it is getting too hot at the plug, this can also cause plug failure.

Last, I would make sure your engine is in top notch shape to begin with. Does it still have good compression? Are there any leaks? Is the engine running well before the glow plug goes bad? Is your fuel in good condition? (Remember the combustable portion of glow model fuels can evaporate, make the fuel a non combustable oil, which is why so many people have problems with engines after storing them over the winter).

Try to take a step by step approach to the problem, and make sure you contact your engine manufacturer if the problem persists. Hope this helps! But most importantly, a $15-$25 tach can help you get the most out of your engine, and help ensure that you are always running at the optimum mixture.