Pardon me for a question from the beginner's section...

I have a couple thousand miles in an AAdvantage account, that I'm hoping to use for upgrades. Do I need to call American and ask them to mail me the upgrade certificates in advance, or can I just walk up to the counter at my departure gate and "spend" some miles on a whim? The reason I say on a whim is because I have some extensive travel ahead of me over the next 2 months and I'd like an upgrade depending on when I'm really tired or when coach is really crowded. So my question is, should I keep some upgrade certificates handy or can it be done on the spot (assuming I want to use my miles to pay).

Thanks.