Either do an upgrade or a new line. Then when you pick up the phone, have them ring it up, sign the contract, and return the phone. Returning breaks the contract and then you can buy the phone full price.
However - not every employee or store will do this. Some look at you like you're insane as they don't know how to do what you're asking. Others know it before you even say it. I've done this for two phones.
Meh, seems like quite the headache. I'll just have them hold one for me for purchase one of these nights