Hey Greg...   Two things...  First, make sure the top is COMPLETELY on all the way around...   Secondly; the skin forming is due to air inside the can.  So, in the past, I have used clean  rocks to fill the space.  After finishing, use the rocks/pebbles to raise the level to the top, omitting the air space.    ~BRUCE~

Before your reseal the can carefully pour in about a tablespoon of thinner.  Do not mix/shake/stir.  The thinner will keep air from reaching the varnish surface and keep a skin from forming. Good luck.  Jer  http://gallery.me.com/jermcmanus  

I have had luck with storing the can upside down.  There is a product that I think is called blonox, that is really just nitrogen, that you blow into the can, and it displaces the Oxygen/Air.  Haven't tried it.

Dave, I believe you're referring to Bloxygen. It's made for exactly this purpose.

Can't you just let skin form and protect the rest of the varnish? Then you remove the skin when you next want to use can, no?