I fly a fair amount in the mountains, though north of your planned path so I don't have any insight on desirable routes. Some good tips already that I'll echo and add a few more...
Winds that are not an issue over flat lands can create significant mechanical turbulence that at best are not fun in a light plane, and at worst can be dangerous. Make sure you check winds aloft.
When flying through a valley, stay to one side (generally the right side, but if winds dictate then whichever side is windward, in order to take advantage of updrafts and avoid the lee side turbulence and downdrafts). It gives you more room to do a 180 should it be required.
Cross ridges at a 45 degree angle instead of perpendicular. Easier to turn away should you need to.
Make sure you know your way so that you don't find yourself trapped in a valley with rising terrain that exceeds your climb capability. Following roads is a good idea.
Weather in general deserves more respect in the mountains. Rising terrain and lowering clouds is a bad recipe.
It's a good idea to have emergency gear suited to the terrain, should you end up on the ground. It can be cold and remote in the mountains.
If you can swing it, get some training from someone who is proficient in mountain flying. At the very least, I'd recommend reading Sparky Imeson's excellent "Mountain Flying Bible".
http://www.amazon.com/Mountain-Flyi...3617615&sr=1-1&keywords=mountain+flying+bible
While mountain flying has elements to be wary of, on a nice day it can be some of the most beautiful scenery you can imagine and is an experience not to be missed. Enjoy, and let us know how it goes!
Circling Mt. Assiniboine at 12k ft...