I have tried different approaches so my fondant doesn't sag and I'm still having problems.  Could someone give me a good butter cream recipe for icing the cake before placing the fondant?  Also  what is the usual thickness for the fondant.  Maybe I am rolling it to thin.  Thanks to whoever can come to my aid.

ADoubt that it's rolling too thin, and I don't think it's the buttercream either. Are you letting your cakes settle before you are covering them? After you fill, but before the crumb coat you should leave your cake to settle at room temperature over night. You could also be rolling fondant too thick

AThe usual thickness is 1/4 to 1/8 inch