This is purely my opinion, I've no scientific research to back this up, just over 20 years of freezing my fingers off through the winter on my bike...
I've purchased expensive gloves, used liners, used heated grips, used BIG handlebar muffs and even tried not wearing gloves. If you're going to ride your bike in sub zero temperatures...YOU WILL GET COLD FINGERS! It's a fact, accept it or buy a car. What you can do is make the whole sorry affair bearable.
The human body is a remarkable machine that can survive many situations. The main part of your body that keeps you alive is your torso. This is where all the important organs live and as such the body will try to keep this bit warm rather than the arms and legs. So if you start to get cold the body reduces the heat in your fingers and toes to focus on keeping your heart and liver warm. So, the very first point of keeping your hands warm is to ensure your body is warm! You might have toasty gloves in protected muffs but if you've no jacket on the body will just suck the heat out of your hands. If however your body is toasty warm then there may just be enough heat left to keep your fingers warm.
Plenty of lots of warm layers. In the winter I look like a big scary biker, the ladies are always so disappointed when after 5 minutes I've removed all my layers to reveal my unimpressively average frame. 1 Tshirt, 2 jumpers, 1 bike jacket, cut off, waterproof over jacket, jeans, leggings, quilted overpants and waterproofs...I can barely move by the time I've ready to roll. It's a pain in the ass but it all helps to keep ME warm which helps to keep my fingers warm.
The next thing is grip. Fearing every ice-laden corner it's all to easy to hold tight to the handlebars. What I find is that this very action makes the fingers stiffen and limits the flow of precious blood to the fingers. When your fingers are screaming in agony try stopping a while and letting go of the bars, the blood returns to the fingers and relief is forthcoming, but it is temporary. You do need to hold onto the bars but try loosening your grip as and when you can. It's not the perfect solution, but it definitely helps. I also find large gloves help, there's room for the fingers to move.
Speaking of gloves, have many pairs to hand (HA!). Wet gloves are the worst thing in the world for cold fingers. Air is actually used as an insulator, most "warm" things are about trapping air, think of duvets, jackets and so on. Water on the other hand (HA!) is often used for cooling. Wet gloves allow what little heat is in your fingers to flow away at an alarming rate. If your ride through the winter you're sure to ride in the rain sometime. I have a motley collection of gloves so when one set is wet I can use another pair. If I'm out for the day and it's wet I may take one or two pairs with me.
My collection of gloves. 4 pairs is still not enough with the rain we've had recently...
Of course things like heated grips, handlebar muffs, hand guards and quality gloves all help. But I find most of these things to be a fix rather than stopping the problem at source. So in summary, be sure your body and legs are as warm as possible, your gloves are dry and don't grip the bars too tight! Still, when all's said and done, you're going to get cold fingers...
