As most of the tires I use are ones that are thrown away at the shop, I usually have rather new tires with holes in them. Lots of things work to boot them to keep tube from pushing through too far and popping:

dollar bill (as stated above)
little section of tyvek (those fedex envelopes, other sources)
a layer or two of cotton hbar wrap (my favorite)
a 1x1 " square cut from a 2 liter soda bottle (sand the edges!)
pretty much any material with a high tensile strengh (won't be pulled apart easily)