Most of the tutorials have been difficult to write, but this one was surprisingly easy. '_' Especially considering that I don't really pay much attention to boobs in general...I guess I just had a lot of innate knowledge.
Anyway, let’s just jump right in with the main point here:
Here is a typical upper body outline for a person without boobs…and a typical upper body outline for a person with boobs.
Notice that the difference is very slight. This is the first thing you have to learn if you want to draw realistic breasted figures— say it with me: BOOBS DON’T MATTER.
Tons of amateurs make the mistake of thinking they have to highlight and emphasize the shape of boobs whenever possible (see: 80% of superheroine leotards…), as if the viewer won’t be able to tell that they’re there unless they make it extremely obvious.
A, I promise, most people can figure it out. B, even if it happens that a certain character’s boobs escape the viewer’s notice every once in a while, IT DOESN’T MATTER. Unless you’re drawing ecchi, I’m sure both you and the viewer have more important things to worry about.
With that said, let’s move on to actual tips:
Fabric Wrapping
I mentioned superheroine leotards before— that’s a great example of what NOT to do. Unless your character has the means to acquire and equip clothing that suctions itself onto their skin, their breasts (and other body contours, for that matter) should not be that well-defined.
In most cases, a simple line or shadow midway down the torso should suffice.
Sizing
90% of the size of boobs should be reflected in the z-axis: that is, pointing towards the viewer. This is often tough for artists to grasp, since it means that bigger boobs don’t look that much bigger unless the character turns to the side. But what’s our mantra? IT DOESN’T MATTER.
Like it or not, that’s reality…if it still feels odd to you, try tracing over photos of real people with different bust sizes. As I illustrated above, the differences from a frontal view will be very slight, if not undetectable.
Fundamentals
Speaking of side-boob, here is a basic shape for you to remember:

A lot of cartoon boobs tend to look like this. ^^ But realistically, the shape tends to come out more rounded, like so:

From the front, they’re shaped more like eggs…or water balloons, for the big ones. Despite what certain artists may have led you to believe, they do tend to sag when not supported by a bra or other clothing. They’re basically big fleshy glands; it’s only natural.

Movement
Boob physics are a little weird…anime and video games aren’t total liars; they do bounce and jiggle around with the owner’s movements, especially when said owner is running or jumping: y’know, vigorous physical activity.
However, in real life they don’t tend to move ‘expressively’, like when a character puts her hands on her hips or sthg.
And they certainly do NOT emote. I read this terrible novel quote once upon a time about a character’s boobs ‘drooping’ depressively…I don’t remember whether it was real or satire, but if any of y’all are actually thinking that breasts react to the owner’s emotions: please, for the love of all that is good and holy, do not. >_<
Well, that’s the end of this one. ^^ Out of respect for the title, please try to keep this thread safe for work in your replies. And as usual, any questions or additional suggestions are welcome.