To some extent, looking like a guy requires masculine looking features, but great things can be done with makeup. In his book Making Faces, Kevyn Aucoin explains drag for girls, so you might want to give that a look before you get started; the transformation he does on Gwyneth Palthrow, by the way, is amazing.
Men naturally have harder cheek bones and jaws than women, so pick up some bronzing powder in a color that tones with your skin and use a fluffy brush to define under your cheeks and jaw. Prep the skin by brushing a little loose or pressed powder before you begin. Skip the blush and lipstick; instead just rub a bit of lip balm on your lips. Keep any other cosmetics minimal, unless your character requires that you wear a certain color eyeshadow or dark eyeliner (all too common in anime and manga).
Lastly, dunno if you're willing to go this far for cosplay, but if you tend to tweeze your eye brows or bleach/wax any facial hair, let the hair grow in. Fuzzy brows and faces are, after all, manly characteristics.