If you can read music (fairly basic in treble clef) then I would recommend trying to get hold of a copy of Kitharlogus The Path To Virtuousity by Ricardo Iznaola published by Chanterelle, No. 730. This is manual of technical development exercises intended for classical guitarists primarily, but the exercises will improve technique for any style. Part of this is a sequence of exercises for improving left hand extension (and contraction also). Despite the method quoted by Rick above (and regards to him from Johnny Collins btw), I personally wouldn't recommend exercises away from the guitar - too often they don't develop the correct muscles/coordination at all. (And no, I don't want to get into a debate about this - that's the reason I normally only post to song threads; life's too short for music and pointless debate)

If you can't find the book, let me know and I'll see if I can send you the exercises.

Mick