I want to welcome you all to the wonderful world of layering. In here it's filled with many different colors, textures, and... layers. But don't rush in too quickly, young lads, for there are a few minor rules that you must adhere to. I'll give you a quick breakdown so that you wont get buried by the wrong layers (ha, I made a funny).
Rule 1: Start small and build your way up.
The first layer should always be the thinnest. It just makes sense to do it that way. How hard would it be to throw a T-shirt over a sweater? Besides, even if you could do it, you'd look utterly ridiculous. Every time you add a layer, it should be a little heavier than its predecessor. Think of each piece having a "grade" or "level" of thickness. You always start with your smaller levels then work your way up.
Rule 2: No repeats.
Keeping with the same "grade/levels" analogy, never repeat grades. It's a waste of time and space. For example, a cardigan has no business being worn on top of a sweater. They're both on the same level and they're both typically made out of wool. To me, it would just look unflattering. Plus, there are way too many other options you have to choose from.
Rule 3: Get textured
This is the time to have yourself a party. I think adding texture creates a more interesting look. For example: here, you have the denim shirt, cable-knit sweater, tweed blazer, and nylon down vest. You can have fun with the colors, too. Here I have a few shades of blues and greens, then pops of red. And for some added character there are all kinds of speckled colors in the blazer (a Harris Tweed trademark).
Denim Shirt~Polo Ralph Lauren (thrifted), Tie~Schreter (thrifted), Cable Knit Sweater~Tommy Hilfiger, Blazer~Lord and Taylor made with Harris Tweed, Down Vest~Gap, Chinos~Dockers, Belt~Dockers (thrifted), Desert Boots~Clarks, Watch~Timex
So there you have it. Three quick rules to layering. I hate when it's 30 degrees out and I go to a friend's house and their thermostat is set to 80.... Thank God for my layers. Or when it's 30 degrees in the morning and 70 by noon, thank sweet Baby Jesus for my layers. Or if you're out on a date and your lady gets the chills, you have an extra layer to lend to her—thank you Holy Ghost for telling me to layer up tonight. So whatever your situation, you should always be prepared with layers this time of year.