The walls of a sukkah should be tied tight so they do not move with the wind6. If this cannot be done, then one should install a series of tight belts or ropes that will not blow in the wind that wrap around the sukkah. The first belt is placed within 9 inches of the floor and the next within 9 inches of the first, and so on, until this series of belts reaches above 40 inches. Thus, with four or five parallel belts, one can create a halachic wall that does not blow with the wind.
6. See Yechaveh Daas 3:46 for sources that any amount of movement will pasul