Why does my butter icing melt so quickly?
Many frostings incorporate more than butter as the fat in the icing. I recently made ones using shortening, coconut butter, and coconut milk solids. These three are all fats that have higher melting points and are more reliable at higher temperatures in comparison to butter.

If you wish to retain as much of the butter mouthfeel as possible, you might consider switching to a buttercream mixture that splices in shortening or even margarine as both are more workable at room temperature. You might also try some of the other variations on buttercream. 

Best practices for icing or frosting in any case are all over the web; 


This extensive article has a real gem to be wary of; keep the butter around 65-70 to ensure you aren't dipping too low or hot. There's plenty of additional information and a listing of strengths and weaknesses of the varieties. 
Here is a run-down of how to frost the cake itself. 





As alluded to in the comments, don't put meltable frosting/icing on a hot cake.
If you are going to keep the frosting in the heat and sunlight, you are definitely going to want to add some shortening or coconut butter in there; that's why it's called Decorator's icing, they put shortening in it so it can hang out at summer weddings.