-If you over-melted your chocolate, you can possibly save the temper by adding in a bit more chopped chocolate. This new chocolate will reseed the crystalline structure in the melted chocolate and help recreate the temper. It's a good idea to always reserve a small amount of chopped chocolate for this purpose. Even if you didn't over-melt, you could always add this reserved chocolate into your melted chocolate without any adverse effects.
-I know others have had great success with chocolate tempering in the microwave, but I've never had luck with it. Even when I use short 5 second heatings, I still find that it overheats the chocolate and breaks the temper. I only use the double boiler method now.