I personally would not recommend getting anything smaller than a 1/2 size down from your Brannock size. Tons of recommendations in this thread as to why, even just a few pages back. As Crane said, there are many techniques to adjust to a size too big but too small? you're screwed. Expensive mistake.
Yeah and I only recommend a half size down under very specific circumstances.
Now I'm going to sidetrack for a second. Filson came up with a really nice coat for seriously cold weather. It's called the Greenwood. It's a heavy Yukon wool coat that's lined in moleskin. It is a seriously warm coat. I wandered around the neighborhood with temps in the teens and subzero wind chills for a couple hours. Nary a chill and I get cold pretty easy. They are expensive but cry once and own a good coat that'll last a lifetime. I have 3 for sale, all are black with a grey black moleskin lining, Alaska fit. 1 medium, one large and one XL. If you want one call the store and yes there's a deal to be had.