Epoxy Before Varnish

[Meridian]

Smith and company (www.smithandcompany.org) make a clear penetrating epoxy sealant (CPES). Made with wood-derived resins, or something like that. Anyway, its very this, goes on extremely easy, soaks into wood. Apply a few coats until it stops soaking in, then apply varnish.
I did this to our raw toerails, varnish lasted longer, and looked good, in the tropics.

[Quickstep]

I've used Interlux Wood Sealer - at least 4 coats - before varnishing. It goes on really quickly and easily. You can't really mess it up. I'm not sure it helps the varnish last longer, but it definitely stops the crazing that you can get before the varnish starts to go bad. (There, I've advertised my 'secret' to the whole world.)
The best protection I've ever seen for exterior varnish is a nice sunbrella cover that goes on as soon as the anchor goes down.