The common practice is to kosher a microwave oven in four steps: 1) cleaning it thoroughly of any residual food resulting from spillage or vaporization; 2) waiting twenty-four hours so that the absorbed taste becomes foul; 3) heating a container of water in it for three minutes (since microwave ovens absorb ĥametz via vapor that rises from food as it is heated); 4) placing something as a barrier between the turntable and the food that will be heated in the microwave, because ĥametz may have spilled onto the turntable.
(The accepted practice for switching between meat and dairy use is as written above. Alternatively, one may hermetically seal all food, or at least one of the types [meat or dairy]. One may also use perforated covers, one designated for meat and one for dairy, every time he heats food in the microwave, which allows steam to escape but prevents it from going into the food. In either of these last two options, one must place a barrier between the food and the turntable. According to R. Rabinovitch, simply cleaning the microwave suffices, even if one did not cover the food.)