I use steel strips, apparently designed for mending or reinforcing joins, at my hardware store for less than a buck a piece. They're made of sturdy steel and come in various sizes from about 1"x3"x1/8" to 2"x8"x1/4".
Tape or otherwise cover the surface, measure and draw the line, line up the steel strip on the mark, and duct tape or clamp it down. Makes for a decent guide. You'll still have to do some finishing work to get things looking really nice, but it does prevent the most flagrant acts of stupidity and/or clumsiness.