Today, I recalled that last winter I noticed an appreciable difference in perceived effort when riding without a coat, so it got me thinking about ways to improve aero through clothing.
For now I've only put thought into my coat, since it's pretty much worn out. My current coat is a Canadian forces winter parka as seen here, it's too long for nothing on a bike as it actually sometimes touches the wheel and gets dirty. This is a pointless aero hit, I would benefit from a shorter coat.
Also, while it was just about perfect for where I used to live, but Quebec city is warmer and so it spends most of it's time open and flapping to the wind, or restrained by backpack chest and sometimes stomach straps, but the zip & buttons rarely actually get closed. So, it catches wind for nothing where a colder coat would be a better choice.
I also note it's puffier then it would need to be, altho you can comfortably wear a coat under this one, but since I'm not spending several hours at -40 it's rather pointless. Especially since a tighter fitting coat would have better aero.
Another thing is that is weighs a few kilos, though this isn't aero related but seems a shorter coat with more suitable temperature rating would weigh less then 1/2 as much, wouldn't mind shedding the weight.
Also, and this isn't winter specific but my current backpack is a sage colored MEC Spirit Velo 25 Daypack. It's quite a fantastic backpack for my bicycling applications, but adjusted comfortably it peaks over the top of my shoulders in such a way as to block my helmet mirror's view, and this square protrusion makes an aero difference that's noticeable by seat of the pants. Not enough to send me buying a new bag but I definitely noticed a difference when riding with a MEC Jet Ranger Daypack. Or rather, I wasn't able to feel a difference with that backpack on compared to riding with no backpack. Thing is that one sits a bit lower and the top part has a strap to compact it flat. With some stuff in placed properly it takes a teardrop like shape and the top part doesn't hurt aero nearly as much. So, bags are something else to think about, aero wise.