I learned to play upside down on my own. It's really not all that hard if you start out that way. I mean...chords and tabs initially have to be translated in your mind into which strings to push down and such, no matter which hand you use, so once you start learning upside down, it just seems normal (although it can really freak some other folks out
).
The main reason I play upside down, instead of restringing and learning that way, is that it means I can play other peoples' guitars as well, and vice-versa, which is a big benefit for me, since I'm often in a place with more guitar players than guitars.
As for learning...I dunno...I figure you've just gotta giv'er; the more you play the better you get. I'm sure there's lots of little tips and strategies I've missed along the way, but the main one is simple: play.