I did a post of my varnish "lessons" and experiences sometime back in the early spring. Others in the group added their "lessons" as well. I think we covered many of the questions you mention (brands, brushes, etc.) with the possible exception of the humidity issue.

One additional trick I've recently picked up is to run a bead of "super glue" around the brush ferrule where the metal meets the bristles. Let it dry at least an hour or so before using it. This seems to prevent much of the bristle "fall out" which plagues cheaper brushes.

Good luck!