Japanese Revelations {Difference between ありがとうございます and ありがとうございました}

This is gonna be a really short post, so don't expect anything extra-spectacular...

ありがとうございます is basically what you say for when someone is GOING to do something for you, or is at the moment you say it. (read: it's present or future tense, just like all Japanese verbs when they're in their plain form.)
ありがとうございました, on the other hand, is specifically for saying 'thank you' for things the person being thanked has already done.
Here are a couple examples:

Person A: I just finished fixing you're car.
Person B: ありがとうございました！

Person A: Can you fix my car for me?
Person B: Sure.
Person A: ありがとうございます！

Of course, in real life, you wouldn't necessarily say 'thank you' in Japanese in response to an English sentence, unless this is so hard to handle that you need to practice it that way. It shouldn't be, though, as far as I know. ^.^

Have fun!