Let me guess, shooting Triple 7?

The penetrating oil technique works pretty good, especially if part of the reason the plug is stuck is because you forgot to grease it (you do grease it when you install it, right?). If the dreaded T7 crud is the culprit, try plugging the primer hole and putting a good amount of BP solvent or plain hot water down the bore. Both will dissolve the T7 filth that will cement a breech plug in place.

One last trick is one you do at the range. After you fire 2-3 shots, stop and loosen the plug, then retighten. This breaks the as yet newly formed but weak fouling seal, and when you retighten there is basically a fault line in the crusty ring that would otherwise seize the plug, allowing it to break free much easier later when it's time for cleaning.

Mike