How to be Insulting at Christmas: Buy crackers without any little gifts inside. If you have the time beforehand, put unpleasant little remarks and observations inside them instead. You might try to glue the paper hats together so that they tear when the guests try to open them.
How to be Insulting on the Beach: If there's enough sand, dig huge walls around your site and try to put your neighbors in the shade.
How to be Insulting in Banks: Try to use one of the automatic cash dispensers, but use it incorrectly. If it's inside the bank, do this until someone is sent to help you out, or until you're asked to leave. If it's outside the bank, kick the machine and try to open it with your car keys, a penknife or your umbrella.
How to be Insulting in Banks: When ordering travelers checks, try to get the smallest denomination available, and then take ages signing each check in front of the cashier.
How to be Insulting at Christmas: Try to duplicate presents wherever possible then lose the receipts so that none of them can be exchanged. If they happen to be things you want yourself, so much the better. Just offer to take them back.