There are so many benefits to being besties; not the least of which being that it opens up a whole new avenue of social not-quite-norms. You’ve got two sets of behavior: things you do around “normal” people, and things you do around each other. Here are some of the things that fit into category number two:
She gave you a key for that reason, right? Because it’s just more convenient for you to be able to let yourself in when you come over. So what if you accidentally forget to call when you’re in the car on the way? She’s always happy to see you!
You share everything anyway, so why would you need to ask if you borrow some of her makeup or that cute skirt for a night on the town? You’ll give it back (you’ll even clean it before you do), and if you forget to she totally has a key to your pad anyway. It’ll be just another excuse for random hangouts!
Very significant other, or your best girlfriend? The casual eavesdropper would simply never know. It’s important to express the sentiment; besides, she’s like a sister to you. How could it possibly be weird or awkward to tell a sister how you feel casually at the end of every call?
There’s only one person who can get away with calling you “bitch”, “crazy”, or “lame”; and it’s only because she says it with love. And you know what? You dish it right out in kind. But only to her; otherwise your life would be much more complicated.
Tough truths are sometimes extremely difficult to identify or face, but your BFF can break it to you in a way that you’re able and willing to hear. And moreover, when it comes from her, it doesn’t sound like she’s judging you; just that she cares about you.