The Puffer Forum

Not so much a trick, but more of a tip, here is a link to a pretty accurate gallon calculator..

Measure your tank in inches and enter it in, and the calculator will do the rest, calculates in US Gallons and UK Gallons.

http://www.fishlore.com/ConversionCalculator.htm

You have to scroll down to almost the bottom it is titled "Calculate Fish Tank Volume in Gallons Based on Tank Size"

They're all over the web. I use this one:
http://www.monsterfishkeepers.com/forums/calc.php