Libra is the sign of peace and diplomacy, so they're the easiest sign of the zodiac to get along with. However, there a few things that can really really get under their skin. Avoid those things and you should have a great time with any Libra.
Realize that since they are ruled by Venus, the planet of love and beauty, Libras like their world to be made up of pleasantries. Your Libra friend will enjoy good food, good drinks and good conversation. If you want to make sure she's having a good time, take her out for a night on the town or a stroll through the park. Just as Libras adore all things that are lovely, they despise things that are crude. Avoid burping uncontrollably or telling tasteless jokes (unless they're accompanied by some healthy sarcasm).
Know that Libras will bend over backwards in order to avoid an argument, so do Libra a favor by not breaking their back. Libras are able to see both sides of just about any issue, and will often bring up the other side of an argument even if it is not what they believe. They have a lot of respect for anyone who can see situations from more than one perspective, therefore someone who has an extreme opinion and is aggressive about expressing it will irritate them to no end. Don't ever try to argue to the point of definitively winning an argument or being right. Libra doesn't believe there's ever one absolute answer, so there's no point anyway. If they agree, it would simply be to keep the peace.
Understand that as children of Venus, Libras can indeed be vain, shallow creatures. They revel in compliments and take criticism very harshly. If you want a Libra to open up to you, or you're hoping to lift her spirits, give her a well thought out compliment. It will put her in a better mood for an entire day. On the flip side, be as tactful as you possibly can while offering criticism. They take comments that strangers make to heart, and take what their friends say even harder.
Know that Libras are very generous creatures, and despise unfairness in other people. Whenever you're in a group situation with Libra, pull your own weight and they will be satisfied. If you're working on a school project, simply do your part. If you're dining with a group of friends at a restaurant, don't skimp on the tip. If you're broke, that's one thing, but Libras absolutely cannot stand stingy people.
Be a really good friend to Libras by helping to guide them through some of their weak points. Libras can be easily manipulated because they tend to assume the best of everyone, and they often have a hard time making up their minds about any number of things. If you know them well and have a discerning mind, they can always use friends with foresight and practicality. However, be sure not to approach them in a judgmental manner. That's a sure way to make them shut down and become unresponsive.