Hi, You keep warm by having a layer or layers of warmed air or water that do not get cooled. Clothing that is too tight squashes out the air and the warmth. Too lose and you get a breeze through. A layer next to the skin that wicks away the sweat is good unless you go the Buffalo route that says that wet is not an issue, wind chill is. Grate wicking layers are made of silk, some of the new technical fibers are also good, but remember if you take a tumble polyaramid fibres etc will melt under impact and fuse with your skin, silk does not, infact any silk fibres that get stuck into abrasions can help with healing.
You want too keep he wind off - muffs are great for this, and layers of windproof are good too - mention has been made of these thin running tops made of wind proof or near wind proof textile. Plastic bags over the feet and hands, bin liner over the torso can work, but tend to cause sweat to stay put and can cause discomfort and when it evaporates - cold.
Ladies tights - preferably over silk longjohns add lovely layers of trapped air and can be real warm. Stockings work, are sexier but more difficult to explain in A&E.
A balaclava inside the helmet can cut out the breeze - but usually are too short. Stitching one to a t-shirt, or the yoke of a tee shirt keeps them in and cuts out the breeze down the back.
Wool is warmer wet than dry, but uncomfortable and you need to cut out the wind.
Really look after your feet and hands, if they are warm you can concentrate so much better - and a handful of front brake with cold hands - eeeek - so layers - snug but not tight with a wind proof layer or two over the top. Mits are warmer than gloves as your fingers are in contact and pass warmth.
Ice climbing bots are over size and have nice - usually - felted wool inners - are really snug, but a bastard to change gear in. A nice set of felted inners would probably work well but you will need to go up a number of sizes.
Friend used to swaer by Dachstein (spelling) gloves and socks etc. WEonderfully warm, bloody expensive.
R