My biggest tip here is that in chore clothes and gear you can be much bulkier and hence much warmer than when riding. I therefore wait until the last possible minute to transition from barn clothes to riding clothes, and I do it in a warm space. Often, I bring down my helmet, riding gloves, and riding boots to the heated part of the tack room and leave them there, then leave him in the crossties to go change into my riding stuff, then get right on.
Part 2: Barn Chores
Women's Wildcat Boots from L.L. Bean: The first thing you should know about my love affair with L. L. Bean is that half my family is from Maine, and L. L. Bean has played a major part in every family Christmas as long as I can remember, even if it's just re-wrapping an old box and thus getting everyone all out of proportion excited before they actually open the box. I love, love, love L. L. Bean.
Ahem. Anyway. These boots, you guys. They are the very, very best. In fact, these are not actually my barn boots; I wear a lesser knock off of these boots every day. These are my shoveling the driveway, walking to work boots. These boots kept my feet warm in -18 on my walk to work. True story. As soon as they are no longer publicly acceptable they will be my new barn boots. They are warm, comfortable, sturdy, and they are backed up by that glorious L. L. Bean guarantee. Lose one in the mud? Pop off a rivet? Gash it open on a stall door? No worries. Send 'em back and they'll replace them with brand new ones.
Smartwool: Yep, here too. Usually wear regular socks, then Smartwool ski socks over them, and wear them for both barn chores and riding. The key for barn chores even more than riding is to have the long, knee-high, extra padded ski socks, because if there's a sensation worse than cold snow down inside your boot and against your bare leg, then I can't think of it right now.
Flannel and Fleece Lined Jeans from L. L. Bean: See above re L. L. Bean. Then go buy these jeans. I will warn you: they fit like your grandmother's jeans. They don't have a ton of give and they are not fashionable. But those factors are far, far outweighed by the fact that these are the warmest and most durable jeans you will ever own. I promise. I usually start off the season in the flannel lined and in the depths of January transition to the fleece lined. Sizing tip: they run small, and if you have any height to you at all I'd recommend getting the Medium Tall or Tall. (I'm 5'9" and not especially leggy for my height, and I need the Medium Tall.)
Gloves: again, this is a hole in my gear. I usually wear mittens over gloves if I'm actually doing chores, not tacking up, but I have no special brand, just some leftovers from skiing days.
Neck Warmer: Same as riding.
Hat: No special recommendation here. I usually wear one I like, which means I'm alternating between my Middlebury ski hat and my Old Sturbridge Village wool hat. The key here is to wear one, because a significant percentage of the body's heat escapes through the head, and to make sure it goes down over your ears. (Also, to remember to remove it and put your helmet on. Don't be like me and get halfway down the aisle every time before realizing that thing on your head is too warm to be a helmet.)
Part 3 next: Experimentation, with a few things I'm adding in to the mix this year but am not yet sold on.