If you're like me, your mechanical pencil runs out of eraser long before lead. Here's a minor trick I use to get a little more life out of a eraser. Maybe other folks have thought of this already, but hey, here it is anyway.
Before the eraser gets so short that you can't pull it out, remove it and stick a small coil of paper in the hole. You have to balance it. Too tall, and the eraser will come out when you use it. Too short and you may as well not bother.