When I fill a cake I pipe a thick dam from buttercream, fill the cake and let the cake settle over night in my fridge (I wrap the cake with saran wrap).
Also, instead of icing the cake with buttercream I use white chocolate ganache on all my fondant covered cakes. The ganache holds up to the weight of fondant really well.
Try and roll the fondant very thin. I like mine to be 1/16" thick. I can almost see though it when rolled that thin.