Problem Solvers makes a stick on cable guide that works without compromising the frame tubing.
http://www.ebikestop.com/problem_sol...f_5-BR4120.php
It's held up after over a year and 3100 miles on my Fuji Touring frame and Nexus hub.
Other than that, I use zip ties at the down tube and between the stick on guide and the bottom bracket.
Under the bottom bracket, the stock cable guide groove was widened, using a Dremel tool to shape it to fit the housing.
I did compromise the frame in a way. I drilled out the down tube brazed on cable stop so full housing could run through it. I installed an inline cable adjuster just below the braze on.