Your under-layer (buttercream or ganache) needs to be smooth (google "upside down method cake" or something like that)..it works WONDERS and I use it with buttercream and freeze for about 2 hrs before removing the cake board.
Make sure the buttercream/ganache is hard (from fridge or still from freezer if covering right after removing the boards). Do let it sit out for a few minutes until you notice a little condensation on the cake. This will help your fondant stick a little as you are smoothing it on and that will help reduce air bubbles.
Hope that helps!