Fishing in the winter can be great if you can tolerate the cold. Dressing properly can go a long ways to making your day comfortable.
Starting with your feet. First cotton socks will not adequately protect your feet. Merino wool is good but your best option is a possum/merino blend. These are the warmest socks that I have been able to find. Unfortunately for whatever reason I have been unable to find them in the states. Google will bring up plenty of suppliers in New Zealand that will ship them to you. Warm feet are worth the price you will pay for these socks. Possum fur is hollow and will trap air and retain heat. Polar bears, wolves and many other arctic species have hollow hair follicles if you doubt the effectiveness of the insulating factor it provides. For added warmth I will wear a second pair of 100% merino wool socks. Next you'll want to make sure your boots now fit properly with a pair or two of thick socks. A couple pairs of socks, stocking foot waders and its easy for you to now need a boot 2 or 3 sizes larger. The last thing you want is a boot that is really tight, retaining heat is all about creating air pockets, tightness around your feet will also slow blood circulations and will result in your feet getting cold quickly. Again don't over tighten your boots when lacing them up, get them snug but not tight.
Pants you'll want to layer accordingly. I like to layer with the heaviest weight stuff I can find. I found some Polartec Thermal Pro's from Cabelas that I really like. First Lite also makes some good 100% merino base layers that I use for the second layer. There is a balance of how much you can layer in relation to mobility. Once you start hitting 3 layers your mobility really starts taking a hit. Personally I like to stay around 2 layers under my waders. First layer being a skin tight layer like the Polartec Thermal Pros, Under Armour also makes a similar base layer. The second layer I like to be a bit looser, again creating air pockets that will trap and hold warmth.
For your upper body I do pretty much the same. First layer is a Polartex Thermal Pro. Next I like a heavy wool vest like the First Lite Springer vest. The reason I like a vest over another long sleeve top is to increase my range of motion so my casting stroke doesn't start getting restricted. Also I haven't had much of a problem with my arms getting cold. The vest does a good job of keeping your core warm. For extreme cold I will put on either a heavy weight hoodie sweetshirt. Under armor makes one that is waterproof and has an infrared liner to reflect and retain heat. This is a good option if you will be wearing a quality wading jacket that will block the wind. If not you will want to replace the hoodie with a high quality fleece wind blocker. Nothing robs the warmth from you like wind cutting through your layers.
Gloves, wish I could tell you of a pair that will keep your hands warm and dry and still maintain feel of the fly line but there isn't a pair that I am away of. Avery makes these neoprene decoy gloves that come up to your upper forearms. I take these gloves and cut the fingers and thumbs off and have been happy with them. Your fingers aren't going to be warm but you should be able to manage. For extreme cold take some hand warmers and use medical tape to secure the hand warmers to the top of your hand and then slip the gloves on. Some of the toe warmers already have a built in adhesive. Take a look at the top of your hand, you'll notice how thin the skin is and how many veins are exposed. The added heat from the hand warmers will aid in keeping the blood flowing in and out of your fingers and hands warmer. I read an article once about how the veins that supply blood to a ducks feet are inter-webbed with many other veins to maximize the warmth of the blood flowing in and out of the ducks legs so they resist frost bite while in extreme cold and wet conditions.
For your head and face a beanie's will retain heat 10x what a hat will. If you still need a hat to cut down on glare, make sure to have a hat large enough to be warn over the beanie. A neck gator will also help retain a lot of heat. Last for your face Avery makes a good neoprene face mask. I really like that the mouth area is really vented so when you breathe your glasses don't get fogged up.
Hopefully you can take a few of these tips and use them on your next trip this winter.