I have a strong proclivity to over-layer. Or perhaps it is just the right amount of layering. Either way it appears a bit excessive. Therefore, as with most fashion topics I have made up a set of rules for myself .
Here are a few of my Layering Guidelines:
+Have an idea in mind
Pirates, Gandhi, Hippies, French Revolutionaries- they all layered. It is okay to have fun with your inspirations. I like to dress pretty extreme- and the people who inspire me are/were extreme too. I can wear an outfit inspired by T. E. Lawrence without actually dressing like him. That is were the creativity comes in.
+Think about your silhouette
You are not a Tele-tubby. please don't look like a rotund blob. If you are layering on top, try keeping it skinny on the bottom.
+ Control your colors
I like to keep my colors simple. I rarely wear fewer than three scarves at a time, and often more than one thin cardigan- so I try to keep things pretty muted and reserved in the color category. When you layer monochromatically you can wear whatever you want.
+Try not to look homeless
Even if you actually are, I would hope that you are clean and well-groomed. Getting crazy with layers leaves people with the idea that you perhaps cannot afford a jacket. Here is a good rule: if you are wearing more than one cardigan you should not wear fingerless gloves. Also- avoid standing near trash-can fires.
(image sources: A, B, C)