If your ganache is runny, leave it in the refrigerator for longer. It will thicken up slowly as it chills. If you also chill your cake, it will help the ganache thicken enough to not drip and pool all down the cake. If your ganache seems too thick, it may be too cold. A few seconds - about 5 - 10 seconds - in the microwave will help to loosen it enough to drip down the cake. The most important thing to do is to stir the ganache well during each process (thickening or loosening the ganache).