What should be the next move in this position? [FEN "2r1r1k1/pb5p/1pp1qb2/3p1p2/3Pn1p1/1Q2P1P1/PP2NPBP/2RRB1K1 b - - 0 1"]


What is the best move for Black in this position. Everything seems defended here. 
What method is applicable in such situations? Method of imbalances?
All black pieces are currently defended but that does not mean there is nothing to do as if you play aimlessly white will eventually gain an advantage.
On analyzing the game the first thing you can notice is that the kingside is completely locked down by pawns so our attack needs to be on the queenside. Ba6 and Be7 are good moves to start gaining control on the queenside. Double rooks on c- file with a c5 pawn break is one option (but it sacrifices the d5 pawn).
Exploiting White's cramped position is essential for any strategy you employ on the queenside side. White will probably outpost his knight on the f4 square so BxN eventually will open up the e-file. Even though this is a drawish game it is far from over and inaccurate play from either player might lose the game.