deadpoetic> I have no tactical possibilities and my opponent has a positional advantage that I can clearly notice.
Well, did you have any advantages? If yes, maximize them while minimizing your opponent's advantages. If no, go out and create some. In a recent game I played, everything was equal except my opponent was about to win the bishop pair on a semi-open board. It's rather dreary playing when everything favors your opponent, so I sac'd a pawn to win his bishop pair. Now he had a material advantage, and I had better minor pieces--the makings of an interesting middlegame. Mix things up. :)
Also, unless your opponent is rated 400+ points higher than you, you almost always get at least one good tactical opportunity. Keep your eyes open!
I am sure we have all been in positions where we are playing with out a plan. More often then I like I find myself in positions where I have no tactical possibilities and my opponent has a positional advantage that I can clearly notice. These positions are almost always in the middle of the game (wouldn't be so bad if its at the end i guess). And unless I had blundered, the material is even so its not like im down a piece.
So the question is what do you do? I can't stand playing these games and usualy just bite my lip and suck it up and play it out. Resigning corsses my mind but being a good sport and all I wait till im obviously losing.