Another simple way to do this is with a Griffin Powermate, a small USB knob type device they made to act as a volume control. It's programable, so you can map it to act in place of the up and down keys(or just about anything else). Just click on the amount field and use the knob to increase or decrease the amount. Allows for very fine adjustments. An added plus is it lets you work two handed, Powermate with one and mouse/tablet with the other.
Started using it with CS2, but they've improved the software and it plays well with LR3. There's also another outfit with an app, Controller Mate, that greatly expands mapability. I think the Powermate is now out of production, but they can be had on ebay. Griffin still supports it on their site.