I used this tape from StreetGlo: http://www.streetglo.net/reflective.tape/

It was $30 for a 2-inch wide, 37.5-foot long roll. There are a range of widths and colors to choose from. One bike frame used about 2/3 of a 2-inch wide roll.

It's peel & stick adhesive. If you apply it wet with soapy water, it can slide around to get a good fit before it sets. It may have a tiny bit of stretch, but not really. I applied it only on straight tube sections as much as possible, and trimmed it to fit the lug lines.

A friend saw my bike and told me about the Bright Bike video. They used 3M's Scotchlite Reflective 680 Series Vinyl, from a supplier called Beacon Graphics, which sells that and some other brands of reflective films in larger rolls at better bulk prices.