I'm a c-cup and I use a compression top like the ones Hindari and Libis recommended; works amazingly!! After struggling with ace-bandaging and sports bras etc etc, this is the most comfortable and most effective that I've found, personally.
Also, keep in mind to pad your waist. One of the biggest differences between men and women is that curve between your hips and your chest.
One last thing to note, even if your character doesn't have facial hair, put a liiight bluish-green tint on your jawline ( unless you're a blonde man, then just play it accordingly ). They still grow hair, but they keep it shaved. But you would still be able to see a hint of where it is, if you know what I mean. If you want stubble, Vaseline and coffee grounds over the blue-green as stubble works strange wonders and it stays on for hours so long as you don't play with it. Just experiment and see what works best with your face and your character.
Just keep in mind the key notes of being a grown man;
• thick eyebrows
• no curves
• posture ( which is easier since you can base it off of your character )
• traces of facial hair
There's always exceptions to the rules, like I said, play by what character you're reenacting and practice practice practice!!