I'm sure my approach will freak out the belt-and-suspenders types, but I just use a carboy brush to remove any stubborn gunk. I think the paranoia regarding minor scratches on buckets and PET carboys is overplayed. The way I look at it, if bacteria can get in a crack, so too can my starsan in order to kill it (this assumes I can actually get my carboy clean, of course). I will slip a sock over the end of my brush on occasion in order to help minimize hairline scratches, though I do that more to keep my carboy easy to see through than I do for sanitation reasons.