One of the nicest sideways-thinking pieces of design I've seen in traffic junctions in recent years has been spiral lanes, which deal with this beautifully. Unless you change your mind while actually on the roundabout you hardly ever need to change lane on one of these.

You select your lane as you approach the roundabout based on which exit you want to take. You then just follow the lane markings, which take you on to the roundabout, around it, and out your desired exit.

Here's a picture (of an unusually complex one; they're usually simpler than this): http://www.whereitis.co.uk/_Media/green-bridge-spiral-rounda...