Everyone deals with this dilemma. Everyone has their own methods that they develop with experience. Usually you have to apply varnish to an area about the size of a few square feet, Cross stroke it to get even coverage and then tip it off (usually with the length of the boat). If the varnish is thin enough and a thin enough coat, you can avoid drips. Most drips are just caused by too much varnish in one place. Sounds almost stupid in it's solution, but that's basically it. I have use both foam brushes and high quality bristle and now mostly use the better quality foam ones. You have to be careful to not push the brush down too hard depositing a load of varnish from the brush because a thicker area looks just like a thinner area until you come back later and see a drip. It's just about technique of application and just try to do light but full and even coats.

I usually mask my boat along the shear line so if any drips should occur, they get caught in the tape at the shear and don't run past that. Then I flip it over and do he other half. An overlap of varnish on the shear never shows.