boil it in battery acid.
other methods i've heard work involve a solder pot filled with sodium hydroxide and potassium nitrate.. and watch your eyeballs!
Chemical extremist?
I've done this before: take a spoonful of NaOH (sold as drain/pipe cleaner or septic tank treatment), in a sacrificial stainless spoon of course, melt it over a flame (an electric or induction hotplate would be better, but the H2O and CO2 in the flame don't ruin things fast enough to prevent its use), and dip the wire end. The enamel burns away and the copper surface (ranging from charred enamel residue to oxidized copper) dissolves off, leaving bright shiny metal. Clean up with copious amounts of water (allow the spoon to cool first, then soak in water -- molten NaOH will spatter if placed directly into water, and NaOH, or solutions of it, will *murder* ocular tissue!), and the copper surface will tin readily.
Of course with the popularity of solderable enamels these days, this is only important on old stuff (I have some spools of brown old-school enameled wire that I use from time to time) and the most tenacious (polyimide and whatnot) types.
Tim