When they ask what your biggest weakness is, give an honest response. Don't say that you work too hard, or that you're a perfectionist. That's BS. Talk about a time that it's bitten you, what you're doing to try to overcome it, and an example of how you've been making sure you stay conscious of your weaknesses and work around it.
Be professional but also yourself. You're young, so they'll wonder about your work ethic, emotional maturity, and ability to handle adversity. I'd keep that in mind and stay centered, don't get flustered, but also crack a joke or two at the right time and make sure they know you're also someone they'd like spending time with every day.
Negotiate salary by phone, and then follow up via email. Don't. Negotiate. Over. Email. Things will get lost in translation and you can lose an offer if you're not careful at this stage. Always negotiate by phone (also negotiate more than salary – bonuses, parking, fringe benefits if any, vacation policy (if flexible, don't push if not), that sort of thing.
Don't burn any bridges, ever. If you ever get rejected, take it in stride, and never say anything (especially in writing) that you can't take back.