Something that won't harm the bricks that can be easily removed. Also I don't want whatever the material is to be visible in a photo. read more
As Larry Pieniazek points out, big elements are harder to lose than small ones, and if you're looking for frequency of loss, an element that's only been in 1 set ever is a lot less common to lose than one that appears in 90% of all LEGO sets. read more