For many writers, this is one of the scariest things about writing a novel: writing the opposite gender.
Let's face it - as much as the world tries to say men and women are the same, we really aren't. There's a difference between "equal" and "the same." Men and women see the world differently. That's not to say one way of seeing the world is "better" or "right" - just that they're different.
So how do we bridge that gap in our writing? How do we go over to the "other side"?
The first step is reading a lot of literature written by the opposite gender. Harry Potter's great - I think J.K. Rowling did a wonderful job of making Harry a real character - but it doesn't help me as much as reading novels written by male authors. Notice how their characters respond to things. You can even look specifically at their opposite gender characters and see if there's anything that seems out of place to you. That's a good clue to where and how they might think differently than you.
The second step is to write. You're going to get stuck forever if you don't just start writing. As you write, you'll discover things about your characters, start connecting to them individuals (not just as men or women) and start to understand how they think.
The third step - and this is the most important - is to have members of the opposite sex read it. Notice that I said "members" - plural. Each individual is different, and what one man might think is too feminine others might not blink an eye at. Take every opinion into consideration, but don't rely too heavily on one person's comments. Don't be embarrassed if all your readers come back to you with the same comment about a certain characteristic, scene, or action by your character. You want that feedback, because you want to make your characters as real as possible.
Fourth, forget it all. This is also a very important step. You know your characters - even if they're the opposite gender of you, you've been inside their heads long enough to write them. There may be some things you need to tweak because of how they're coming across, but you should understand them better than anyone else at this point. Now, this isn't to say that you literally forget what people are telling you - if all your friends think the male love interest in your romance novel is gay, that's a problem - but have confidence in the character you've created and that you know who they are and what they want.
Now, here's a quick don't: don't bog yourself down with psychology books and the like. You generally know when something will be viewed as "feminine" or "masculine" - follow your instincts, don't over-think it.
Yes, men and women are different, and those differences should not be ignored. However, do not let the fact that you are a woman keep you from writing from a man's perspective, or vice versa.
Do you have any MCs of the opposite gender? How did you approach writing them?