I'm a really big fan of battery erasers! They can provide more friction in a tiny area than using a block eraser by hand. The eraser can be cut to a fine line with a craft knife or ground to a point with sandpaper for more detailed use. I started with the Derwent battery eraser, and it is good and sturdy. You can see it in use HERE
. There is another longer style that is branded by several different companies (I've gotten one at Hobby Lobby and one on Amazon), and those seem comparable to each other and are also good tools. The erasers are a little softer, but they have the option of a smaller diameter eraser, and the motor spins faster than the Derwent model.