Be mean, rude, or otherwise unbearable to the non-agent staff in the office.
Yup, that'll do it.
There are three people in our office who you might think of as "assistants." They are not. They are godsends. They are incredibly valuable, and I treasure them. These are the people who sort the mail, run the manuscripts through the xerox, prepare the UPS shipments, order the office supplies, and answer the phone. They are worth every penny they get paid, and should be paid a lot more.
Anyone who is rude to them is foolish and shortsighted. Given a choice between them and anyone else, I pick them. They make my life easier. It's not a hard choice.
They get to put up with exactly as much crap from people as they choose. If they don't want to deal with you, you're fired. As a client. As a prospect. As an anything.
If you want to yell at someone, you can yell at me. Don't even think about yelling or being rude to anyone else. You're replaceable. They are not.