Flip through magazines, watch television. List out the ads you like most and find out which agency made them. Find out which of these are working for your competitors and so are unavailable to you. After the shortlisting, choose the one you think suits your needs. Arrange a semi-formal meeting with the head of the agency and creative director. If the relationship turns out to be mutually beneficial and cordial, initiate the next phase, i.e drawing up your contract. Don't haggle over the price your agency demands. Negotiate as you would with your lawyer or accountant. Insist on a long term contract. All this will ensure that your agency does not ditch you for your competitor. Keep a tab on the working of the creative team to make sure you get what you want. But treat them with kid gloves-- it's creativity that's involved! If your account is too small to interest a big ad agency, advertise for an experienced copywriter and visualiser, who can work for you on a freelance basis.