For experimental purposes, an electrode array or comparable 2D device can be mounted and re-mounted on the same spot by using visual markers, e.g. with water resistant ink. By using a matrix of actuators with a back and front, and a clearly defined "up and down" (and/or left and right) one could suffice by just two little spots on the skin.
Alternatively, stable bodily features can be used when applicable. For example ribs and vertebrae are stable positions that can serve as markers.
A more sophisticated means is a stereotactic procedure. For example the head can be easily fixated using a chin rest and head support on the forehead. This pretty much fixates the head. This is for example used with repetitive transcranial magnetic stimulation, as shown in the figure below:
Although this list is not exhaustive, it gives an idea of possibilities.