Applying polyurethane to a vertical surface?
Preventing runs is as easy as following one simple approach: Start heavy, finish light.

What that means is that you immerse your brush to maybe inch of depth, scrape off one side on the can, and apply the remaining product load to the project in a new area adjacent to the previous work. The idea of "starting heavy" is that you apply enough product to thoroughly coat and fill the texture of the surface. 

You then brush to distribute that load, working it outward until you achieve a uniform film that's blended with the previous area. This will not run. Finish with long strokes parallel to the grain. Do not work the finish so long that it begins to dry and get tacky.

With a bit of practice you'll be able to apply a uniform, drip-free coat to surfaces at any angle.