Here's a tip for you li-po fans, instead of lead you can use tungsten weight (props to rcarmchl). Tungsten weighs 1.8 times more then lead. It is usually found in the Pinewood Derby car section at your hobby shop. Available in strips, small cubes, and also as a putty (made by Pinewood Extreme). Small enough to put anywhere just attach using Shoe-Goo. The one draw back is it is expensive, $10.00 for 2 ounces (about 56 grams).