How to come up with plans in a dull position?

How does someone come up with a plan of action in a position devoid of action?

There is always something to do in any position.  

The term "dull" makes no sense, without an example of what you are talking about.

In some positions, no long term plans are possible.

In those cases you typically make small improvements to the position. For example if a piece controls a few more squares somewhere else, you might make a 2 move plan to move it there.

If, after the logical progression of the position, something will be weak, you might take the time now to improve it. A simple example is luft for the king so there won't be a back rank mate. If the weakness is for your opponent, you might take the time now to make it a bit worse. For example you'll eventually transition into an endgame where their dark square bishop is hard for them to activate, so you put some of your pawns on light squares so it will have even fewer targets, or you put some of your pawns on dark squares to restrict it further.

If there is nothing to improve, just proceed with the structures changes (pawn breaks) and piece trades that are inherent in the position and try to press for a win after the position has transitioned to something else (which may be the endgame).

If there are no weaknesses, no improvements, and no transitions, then it's time to start shuffling pieces back and forth and offer a draw