Buffy has taught many valid and true life lessons. Others are...questionable. But that's sure as heck not gonna stop me believing them :)
1: The British wear Tweed. All of them. If you don't have at least one tweed outfit check your nationality, because you're apparently not British.
2: Your friends can forgive you for dating a vamp who has tried to kill them, running away from home, believing they aren't real, and even trying to feed them to a demon. But they wont forgive inability to lead an army, so brush up on your commander skills while you have the chance.
3: When English people decide whether to be good or evil it alters their accent. It doesn't matter where about in England they were raised, when they're good they are upper class English and when they turn bad they become cockneys.
4: When it comes to sex, horror movie rules apply. If you have sex something terrible is going to happen. seriously. Every time.
5: Leather pants and all who wear them are evil.
6: No matter how many times your friend is right when they suspect someone is evil it is still okay to doubt them every single time they do.
7: It is not considered fickle to nearly end the world due to grief at the loss of your lover then start dating again a few months later.
8: When someone you love dies don't worry. You'll probably see them almost as often as you did before.
9: If your friend returns to their former evil ways and kills people try to kill them with a sword. Even if they became human in the first place through the simple act of destroying their amulet, so making them human again would be easier than attacking them with a sword. Don't even mention the easy 'destroy the amulet' thing. The sword plan is cooler.
10: You will only get in a fight with the type of demon that can only be killed in a specific way when there is luckily someone around who knows how.
11: Clawing out of a grave through 6 feet of dirt will leave a slight smudge on your face. The rest of you will remain perfectly clean.