I find my annual November-December hiatus is sufficient to recharge through the next year. Been playing off and on (mostly on) since '89, and never suffered in-season burnout.
Some factors that help:
*limit number of leagues (I'm in a career-high 2 leagues this year)
*be competitive in a competitive league (2nd and charging in my 14-team H2H; 1st and trying to hold off a charge in my 16-team roto ... I'm looking at you, NZF
*include, um, incentive (even a modest payout can do wonders)