The glow plugs stick in 2 places or both at the same time. Never-seeze type products can help in the thread area but to prevent them from being trapped by carbon I think you need to keep the fuel injectors in top shape (of course the GP holes should be cleaned some how when the GPs are out). If your injectors (and air filter) are working perfectly there should be less carbon = less chance to trap your glow plugs.
I think removing the GPs every 6 months might be excessive but would alleviate the problem of them becoming stuck. If I had one of these type of cars I think 1 time a year just before winter would be a good time.

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84 300D, 82 Volvo 244Gl Diesel