Know the scams and what to avoid. Don't pay reading fees or jump to bite if a contract comes asking you for money. Don't plaster the copyright symbol all over your manuscript. Your work is already automatically copyrighted. Do show from your correspondence or phone conversations that you are an agreeable, flexible person.
Agents take personality into account when deciding whom to represent.
No one wants to work with someone who is unpleasant. Don't burn any bridges. No matter how frustrated or angry you are, it will not do you any good to vent.