Practice. You have to pretend your tier is NOT going to be covered in fondant. Get it is as perfect as possible before you put the fondant on. No bumps pre-fondant...no bumps post-fondant. Work with a cold cake (not cool, not in the freezer for a few minutes, but cold. chilled thru. in the fridge for 1 hour or longer.
Roll your fondant to 1/4 or even 1/16th (that's what I do) thin. Use two fondant smoothers at once. Practice and practice some more. Trust me. I do not tolerate the slightest of indentations, bumps, air bubbles or anything else. 
How thin are you rolling your fondant? Make sure you don't roll it too thin, and the icing under the fondant makes a difference too, make sure the icing is smooth first... if it has any bumps, the fondant will too.
Sorry if this is a dumb question and sorry to hijack the thread but does fondant stick to hard buttercream or do you need to put another layer in between?
In the Peggy Porschen book, she says to put a layer of Marzipan in between the buttercream and the fondant but I don't want to do that.
You aren't trying to smooth the fondant with your hands, are you?
If so, you should get ahold of a fondant smoother. Wilton makes one, it's about $3 at Wal-Mart or Michael's or anywhere that sells Wilton. It's a little plastic piece that kinda reminds me of an old school iron. It's white, flat and has a handle on the back. Use that to smooth your sides... it gives more even pressure and doesn't leave grooves like your fingers do.
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