Well... I don't know as I have any particularly great ideas for you. But one easy way to make yourself look more masculine relates to what you do with your hair. Either comb it straight back, get a brush cut, or perhaps shave it all off entirely. And then, if you wear glasses, find a heavier more masculine looking frame.

I suppose it's possible you might also be able to do something in terms of working on your facial expressions so that you have a more stern look about you. (Law enforcement officers must get some sort of training like this so they display an air of competency & control.) At least these are the things that occur to me.