Megan - from St. Louis, MO
First, make sure your cake is completely cooled before spreading on the icing – any hint of warmth will melt the icing, causing it to run down the sides of your cake. Then, make sure the icing isn’t too runny – it should be thin consistency for spreading, but it should still hold a peak if you pull up on it with the spatula.
Also, in hot, humid weather, you may find your icing melting. Make sure you keep your iced cake as cool as possible, and never place it outside or in direct sunlight for any length of time. Buttercream icing can be refrigerated, so if you want to be extra careful, certainly place your iced cake in the refrigerator.
If you live in a high-humidity area, you might want to try one of the two High Humidity Buttercream icings on our website, "High Humidity Buttercream Icing I" or "High Humidity Buttercream Icing II"