The basic principle is - lot's of clothes. There - see you in 2018 :-). Well, OK, there's more to it. The basic principle is of course layering. Have something on your skin that provides a little warmth and still wicks and transports your sweat away to keep you dry. This is especially important on your feet. Wool is good (merino in particular), but the most efficient in sweat transportation is polyester (which quick begins to smell like a dead wildebeest). Wool is warmer.
Somewhere in your layering a bib is essential! Whether it's directly over your inner layer, or just under the waders is more or less irrelevant, but a bib prevents cold from creeping in if pants and a sweater comes apart on the waist line. My bibs are fleece and not particularly thick, so I add a second layer of fleece, and if it's really cold, look towards the gear developed for diving. A good friend, Allan, recommended gear from "Fourth Element" a long time ago, and I have a pair of their pants, and they are by far the warmest fleece I have ever had. The diving gear is developed to prevent compression of the fleece - and the less it compresses, the warmer they stay.
With an inner layer (merino), bibs and my Fourth Element pants, I can stay warm for a long time in cold water.
On my feet, I wear polypropylene socks and to layers of wool over them, which also keeps me warm for a long time. Make sure there's room enough for the socks in both the neoprene foot of your waders and your wading boots, otherwise you'll freeze quickly.
On my upper body I usually wear a little less, since it's not under water and not getting compressed from water pressure. An inner layer of merino or polypropylene, the upper part of my bibs and then a fleece (thin or thick). The outer layer is my wading jacket, and this is usually enough to keep me warm. A jacket is important as this is your windproof part, which is essential. And by all means - use a jacket with that'll close watertight around your wrist. It's not fun when water runs up your sleeve when releasing or landing a fish! As you can see in the PoD, my friend Morten even buys his waders so he can fit a DSLR with a long tele lens inside them - and under the jacket.
The head is important to keep warm too, but much simpler - usually a thick(ish) fleece hat en enough, and if not, I use the hood on my jacket. I almost always wear my cap under the hat, just because I can't fish without my cap.
Gloves? I don't use them - I can't get used to handling line or rod with gloves on, but some do - they all get wet though, and more or less loose their warning ability anyway.
There are a few more important factors. If you fish actively (move around a lot) it's infinitely easier to stay warm, and if you can get away with it - don't wade deep! That also helps a lot.
In 1 or 2 degrees C cold water, everyone will get cold at some point, and if you do get cold, get out of the water and take a brisk walk on the beach - with the amount of insulation you're wearing, you'll quick be warm again.
And - EAT! Eat something with lots of energy in it! On our first morning on Rügen, we went out in the morning without breakfast (since we'd forgotten to get some) and I was cold within 45 minutes and had trouble getting warm again. After lunch - no problem (apart from my feet as i had forgotten one of the pairs of socks). And bring something to snack on if you're in for a long day - it helps. Also stay hydrated - even though it doesn't feel like it, you still need it. But balance it so you won't have to run for slash every 30 minutes - it takes forever to get through all the insulation :-). If you feel like it and have the opportunity, bring some warm to both eat and drink. There's nothing like a hot meal on a cold day. Spaghetti bolognese does really well in a thermos if you're less inclined to bring a stove.
All for now - have a very Merry Christmas and a Happy New Year to all you. Thanks for reading my ramblings in 2017. If you disliked them, I'm sorry to let you know that I'll continue in 2018 :-).