Tips:

1. Don't be shy about getting your client to sign a contract. Getting their signature is no guarantee they will pay but it does increase your chances.
2. Should a client not pay within the time you specified on the contract, send a formal, friendly letter along with a copy of the contract they signed. Be sure to reference the contract and the fact that your payment terms were outlined on the contract itself.
3. Having this piece of paper is not bulletproof protection. However, it can help you tremendously should you ever run into a client that refuses to pay. If someone has breached a contract, consult an attorney. As a freelancer, just be sure you've exhausted all efforts to get the money you earned. Otherwise, the client could spread the word and other agencies may be leery of doing business with you.
4. Consider putting a line in your contract that you will get paid even if the project does not make it to print. A client could say the project was killed or changed so that your copy wasn't usable. And this results in you not getting paid for the work you have completed.
5. Ad agencies and business owners are not bad people! Contracts are in place to protect you just as much as they are to protect them. It's rare that you will find someone who won't pay for the work you completed. The contract can help ease both parties to know you are going to complete the project you promised the agency/business and they are going to pay you for your hard work.