In a reversi game the wall seems to be a good place to stay on but do not take more than two wall otherwise your opponent can wedge easily. 

Only create a unstable wall when there is a solid line created by your opponent next to your wall.

When you lose control of the middle try to control to wall and make your opponent come to you because then he will expand from the middle and end up taking more disk. In this way you can isolate him easily.