1. Don't be a brown-noser
It's one thing to say hi to everybody you meet; it's an entirely different matter to attempt to leap-frog over your colleagues by proving just how much you have in common with your superior by hanging onto their every word and making it known that your life is made every time they speak to you. Plus, people wise up to the act after time, however much they may enjoy the attention initially.
2. Know when to keep your nose to the grindstnoe
Even if your internship is unpaid, like all of mine were over a period of six years, that isn't a free pass to idly chat to the staffer next to you, or refresh Instagram all day. You're there to learn, however it may feel as though some days, it's a waste of time (and I guarantee you, some days will feel like an elongated time warp.
3. Don't dress better than your boss
Who do you think you are? By all means, wear business casual if your boss wears filp-flops and crusty band tees if you are at a startup, But one needs to be conscious of the fact that they need to maintain a somewhat professional demeanor especially if they want to be remembered in the long run positively. But never arrive every single day in a double-breasted suit with a pocket square and wingtips if you want to be taken seriously for your work, rather than who bankrolled your ridiculous otufit.
4. Know that everyone has their worth
There's something to learn from for everybody. There may be a class divide between you and your cubicle-mate, but that should not stop you from extending an offer of friendship or rebuffing their advances. Again, you are an intern: who do you think you are?
5. Be nice
Help people out! If someone is starting to become flustered because they don't know how to do something, does it really take that much of your time to waltz over there and teach them a lesson they'll remember for some time? This gesture could pay off in dividends for you, but most importantly, people will always remember how you made them feel, not necessarily what you said.
An internship is the first step into the corporate, 'real' world. Good luck!