What are my plans? In the middlegame, you must have plans. Are you going to use pawn levers to gain space or destroy structure? Where? Are you going to shift pieces to attack on one side of the board? Can you find ideal placement for your pieces that aid in these plans?
If the center is inactive (closed or neither side wants to change it), then a plan on one side of the board or the other is essential. If not, then basic opening principles can apply to the middlegame such as: control the center, gain space, activate pieces, etc.