Its not just your feet its the full package, but starting at your feet, you need a good pair of socks, that are part manmade and natural you wont go far wrong looking at ex military socks, they need to be loose fitting, with no constriction at all and that includes the calfs when you pull them up. A lot of anglers swear by Neoprene and seal skinz, all they do is make your feet damp thus allowing the cold in.
Boots i use a good hiking boot that's a size bigger, i also check the sole that there is good insulation, not just the insole but the sole itself, one i have my eye on is the TFG AP extreme, for wellies i have a pair of muckboots Tay sports, ensure you have enough room any constriction of your blood flow will only help the cold get in. On long sessions i take a spare pair of socks and regularly change them when my feet are getting damp, if they are getting cold give them a rub as that will get the blood flowing to the capillaries. Your feet will sweat no matter how cold they are.
I wear a pair of snugpak salopettes over some long johns, they are fantastic, these will keep your midriff and legs warm, a quick look in the ski section at TK max is worth a try for some bargain salopettes. Before i put my jacket on i also wear a Body warmer, this will keep your core extremely warm.
As you can tell its a full package, sort your feet out first and work up.
And dont forget your hat.