Kawaii Pocky: For Underworks, buy on the range that matches your measures. The compression shirt will be hard to get on, but that just means that it's going to flatten you out. There are methods (some in this thread, I think) that should help you get it on and off, and eventually you get used to it. I have an Underworks 997(Double Front), which I love, but if you don't want to deal with the extra length their Tri-top gets a lot of good reviews.
Being a queer, queer child, I've been working on perfecting the ability to transfer, in only a second or two, between masculine and feminine body language. I have fooled some of my best friends when they aren't paying attention. So here's what I've learned.
Tuck your butt under you. This is a trick I picked up as an equestrian, but if you can tuck your butt under, it lessens its impact. It'll feel a little weird when you first start doing it, but it helps. If your knees are bent slightly, you're doing it right
Slouch. If you can get away with it, dear God, slouch. It's pretty masculine-looking, and it can detract from your waist. You don't have to slouch a lot. Just roll your shoulders forward a little. Curve your spine.
Spread those feet! About shoulder width is good-- it'll feel weird, especially if you try walking with your feet so far apart, but keep in mind that your shoulders are narrower and you're making up for not having dangly bits.
Weight in your heels. This comes fairly naturally for me, as a result of tucking my rear end under. It seems to help--maybe it has something to do with centers of gravity? I'm not certain at all that's the reason, but it seems plausible to me.
If you need help with mannerisms, honestly the best place to start is FtM forums and sites. You'll get a lot of information on voice, posture, mannerisms and you don't have to dig around too much.
Izaya Orihara: 95%
Rokudo Mukuro, TYL: 85%
Xanxus, TYL: 75%
Fushimi Saruhiko: 50%
Agatsuma Soubi: 10%
Scar gijinka: 10%