Put an electrolytic capacitor from the sensor lead on the gauge to ground. 50volt / 100uf should do it. you can go to a 220uf or 470uf if you want a greater lag. (uf stands for microfarad) You can even go bigger if you want like a 2200uf, depends on the resistance of the sender.
The capacitor is polarity sensitive so + goes to the sensor terminal. Radio shack should have these.
http://www.radioshack.com/product/in...ductId=2102506