When I really can't find a plan, I improve my pieces. Is there a square where your knight would rather be? There's almost always a hole somewhere, where they can sit nice and pretty and annoy the opponent, so find a path while you're twiddling your thumbs and reposition. The same is true of all the other pieces, but often the knights are easiest to improve.
Think about where your rooks are sitting, also - is there a half open file you can guard? Going opposite the enemy queen is almost always a good improvement. If you've got blocked bishops, see if there's a way to unblock them. That doesn't just mean moving them, but also forcing your opponent to move his pawns out of the way.
I hope that gives you some ideas! I actually enjoy complicated middle games now, because I find that my opponents are often a little more clueless than I am about how to proceed. If you make small improvements to your pieces, you'll be way ahead at the end.
Ever get into the middle game without any clear way to proceed? So you sit there and study the game but without seeing any really good move or combination of moves, still wondering what move to make next? Seems like I get this way when the pawn structure is taking up the middle of the board and they're blocking any advances.