I just had my first real experience with this problem last night, and figured it'd be more constructive to raise a thread that already has a lot of good ideas in it that to start a new one.
As Denmark in 1753, I couldn't capture a single province from the Swedes. Hundreds of thousands of men, thousands of cannon, only sieging the southern (non-winter) provinces, and still zip. I appreciate the fact that fortresses should markedly slow the progress of war, but from what I can tell, unless I'm just missing something, they seem to stop it altogether at this stage. The makes late game wars, well, less than interesting and more than a little nerve-fraying.
Anyway, I figured I'd bring this one back up for discussion. While the "support cost" idea is my favorite, I think it'd probably screw the AI up something wicked. Anyone have any clever ideas?