Standard tire levers work well, and some even can do it without the use of a lever. But I use
http://www.rei.com/pwr/product-revie...eed-Lever.html
Just wedge between bead and rim, swing down, open to full length and slide end onto wheel spindle. Pull the unit completely around the bead, like the hands on a clock. It is a little long to fit into an under-the-saddle bag, but it does fit neatly into a jersey pocket. I have mine attached to the rear of my seat post by the saddle bag strap.