Okay, fifty-somethings, how do you keep your weight down, especially in the winter?

My weight fluctuates in a ten pound range, from 180 lb to 190 lb. Lowest around August, highest around January. No matter how I try, I cannot get below 180 lb; that weight requires a whole summer of riding, eating meat and fish and vegetables and egg whites and other low-carb stuff, only one beer every other day, averaging around 1600-1700 cal/day except on long ride days. Then after Labor Day, bread and pasta and rice slowly sneak back into my diet, I find myself stopping at the pub more often for the tasty fall ales and stouts, and my weight starts creeping up. By the time I get alarmed, I'm in the mid 180's and momentum takes me to near 190 lb.

This is really frustrating to me. I'd like to break this cycle, get down to 170-175 lb over the winter and start the new cycling season lean (for me) and mean (for me) and wearing medium Castelli instead of large Castelli. At 180-185 I look and feel "okay" but definitely could still lose some padding. I'm 5' 11", medium build.

What do you suggest? What would you do, in my shoes? As I've gotten more, err, mature, losing weight and keeping it off is no longer easy; it is getting harder.