Breasts are, in my opinion, one of the more difficult aspects of the human form to convincingly capture in pixel art. Anatomy is a massive topic on its own, and I would recommend reading some traditional art books on the topic if you'd like to explore further (I like this one,) and there are also some great free resources online. There's no replacement for experience, so you'll probably have to study and draw a lot of breasts before it really starts to click (at least they're nice to look at.)

The following is a graphical mini-tutorial I created with Twitter in mind, which covers what I believe to be the most important things to consider when pixeling a nice set of milkers. Follow me on Twitter for more visual accompaniments to my writing, and stay tuned for more long-form articles in the near future.

Boob Anatomy: The nipple is often used as a measuring landmark for drawing the human figure—falling about one head length below the chin and one head length above the navel. However, the mass of the breast often causes the nipple to fall below this point. The breast mass is built on top of the pectoralis major muscle of the chest, which inserts at the armpit. It's very important to make sure this insertion point is not drawn too high or too low on the figure—otherwise, your boobs may look “off” no matter how well-rendered they are. The armpit falls about ½ head length below the top of the arm/collar bone. (Keep in mind that pixel art characters will often have a head that is significantly larger relative to the rest of the body, and in this case “head units” are used as a more general measurement tool relating to the rest of the figure, rather than referring literally to the size of the head of your sprite.) Because of the rounded underlying structure of the rib cage, breasts are typically angled about 45 degrees outward, which means that the nipples also tend to point out (rather than being centered on the front of the chest.) Because the breast connects at the armpit, it will follow the movement of the shoulder. In my experience, it is very easy to underestimate the effects of foreshortening on the further breast.
Boob Diversity: While there are many types of breasts, no breast is a perfect orb (unless it is a bad implant.) Overall, the breast forms a sort of teardrop shape, with the mass tapering higher on the chest. There is some space between each breast. In reality, they will never press together while unsupported. Generally, breasts will sag more as they become larger, but some smaller breasts also sag more severely. There are many types of nipples as well! (However, the resolution and style of your pixel art may not allow for total variety in representation. You may find more freedom in representing position and color rather than size or form.) Technically, the term "nipple" does not include the outer areola. Here, I'm referring to both.
Bonus Boob Tips: Try softening or breaking your line where two breasts meet or a part of the breast merges with the torso. The areola often has a smooth transition into the skin, so you may not want a hard outline around the entire shape (or at all.) Because the breast is supported by layers of fibrous tissue (to varying levels between individuals,) a natural breast does not actually behave quite like a pillow or water balloon. You may often want to exaggerate the softness or squishiness of the breast, but that is a personal artistic choice. Unless it's molded to the body, fabric will never cling to the separate breasts in this way. It stretches between the two, often causing wrinkles across them. (Your pixel art may not have room for this detail.) Breasts are usually asymmetrical, so you can use this as an excuse if yours don't quite match.