I'm sure I'm the last one to find out about these, but I figure I'd post it. I've always been annoyed by the lack of damping in aftermarket fuel gauges. I tried this and it was well worth the money.

Fuel Gauge Stabilizer, 356

It's just a 0.1 farad capacitor you wire between the signal wire and ground. It does not provide a ton of damping, but at least the needle no longer reflects every pebble I drive over.