"By producing a full, completed page a week, you will get better faster than you ever thought possible. So plan out your series, do your concept art, write your script, definitely do all that preparing stuff... but you MUST START."
(Seriously though. My style has been stagnant for over a decade, and in the 40-ish pages I've drawn so far, I've seen actual improvement. Or at least, I've been far happier with the most recent 10-or-so pages than I am with the 10-or-so earliest pages.)