I think transwomen neglect jackets, which is a shame because a jacket can really pull a look together. Put on a t-shirt and jeans, and you will look OK. Add a jacket to that, and suddenly you're amazing!
Overall Fit
I love a tailored look, which means walking the line between wearing a jacket that looks like a box and one that is too tight. If it pulls across the shoulder blades when you first put it on, it's too tight. If it feels OK, but when you hug someone, the seams start to pop, then it's too tight.
If you have a jacket that passes the shoulder blade and hug tests but is too tight to button, just leave it open. The open jacket look is nice and acceptable everywhere, even in most workplaces. If you crave a jacket that closes but can't find one that looks good, try a knit jacket. Knit jackets can have a structured look and are stretchy enough to close even over large boobs.
If you have broad shoulders, you are stuck with larger jackets. Inevitably these will be too large at the waist, and you'll look like you're wearing a box. There's an easy solution: Get a tailor to open up the lining and add two darts in the back. They should start right behind the sleeves and end just before the hem. This will run you about $50. You can do it yourself, but it requires some skill.
Some tailors will refuse to do this when the jacket is really structured. They were taught in tailor school that the right way to do this alteration is from the side seams. I ordinarily trust tailors, but the problem is when you adjust the side seams, you almost always have to move the pockets. This becomes a huge deal and a much more expensive alteration. So while they are undoubtedly correct, my method is a quick fix and makes the jacket look like it was made for you! So if you can't talk your tailor into it, keep looking until you find one who will do this for you.
If you have broad shoulders and small boobs, jackets with lapels will sag in the chest area instead of making a nice, clean vertical line. That creates problems for the entire bust, arm, and shoulder area. While this looks terrible, there's an easy fix: Just sew a tiny snap one to two inches above the top button. This will make your lapel lie straighter, making the whole jacket fit much better.
Shoulders
Most jackets have shoulder padding. Transwomen are averse to shoulder pads because they think they will look less feminine. Some of this depends on your build, but a small amount of padding is usually not a problem, and it gives the jacket its shape. But both the pads (and the seam) should stop at the end of your natural shoulder. If they go any further, the entire jacket will look too big. Getting a tailor to adjust the shoulder is a huge, complicated, and expensive alteration. It's easier to buy a jacket that fits well on the shoulder.
Sleeves
Unlike men's jackets, there are no rules about the sleeve length for a woman's jacket. A common (and very nice) length is about one inch above the knuckles and one inch below the wrist. I like my sleeves higher to show off some pretty bracelets, so I get sleeves that stop about one inch above the wristbone.