Found some information to hide your chest, but it really depends on your size >< Sometimes if you don't wear a bra, or a small sized sports bra, it works. But if you have big ... ones, then you can try tying and binding (isn't as painful as it sounds o.o) So here goes:
There are two major factors involved in binding:
1. Breast density
2. Breast and chest size
and (at least) twelve ways to bind:
Things (More Or Less) Designed for Binding
1. The Frog Bra, a sports bra from women-owned Title Nine Sports in Berkeley, California; their catalog boasts, "It's almost like not having breasts at all. Well almost."
2. A regular sports bra, sometimes in a too-small size.
3. Abdominal binders, which are designed for abdominal or rib support.
4. Men's compression undershirts, designed especially to flatten out the curves of a man's figure.
5. Corsets.
6. Diving belts, the European kind which are wide rubber objects.
And Others
1. Plastic wrap, shown off to great effect in the book Boys Like Her by Canadian performance group Taste This
2. Ace bandages
3. Control-top pantyhose with the crotch and feet cut out so that the crotch becomes the neckline and the legs are sleeves
4. Wearing lots of clothes (especially an open jacket, a vest, a heavy coat, or an overshirt)
5. Duct tape
6. Packing tape
Go to this place to see more. http://everything2.com/title/How+to+bind+breasts
Hope this helps a bit!! I am planning to cosplay Kanda from D.Gray-man since I'm pretty tall. But my size isn't so big so it's not a problem if I just wear a sports bra x)