It helps when you have a watermark on your artwork, but like mentioned, this thing hardly needs it.
Where you need to get this across is verbally when you initially talk to the person. If you still want to get paid for artwork, you need to tell them upfront before anything else.
After that's accomplished, you send them a quote for the work being done and explain that all artwork is your property until it is paid for in full by the customer and you release it to them. Once the deposit is given, then you start your designs/layouts and when they accept it, you collect the balance and if it suits your requirements money-wise, you hand over the written rights for them to use it for anything down the line.
You do realize, that by getting an hour's worth of wages, you are handing them full rights to use this artwork for anything from television commercials to billboards to T shirts. Had you gotten your upfront cost and not put a bad taste in their mouth to do business with you, you could've quite possibly had all their work from here on out.
When you have a method of doing business and follow your own rules, it's kinda hard to mess up. Doing this stuff haphazardly only cheapens your worth and the worth of the next guy doing business with this guy.
Had you a set of shop policies, and this was indeed you method of operations, you wouldn't have to ask fundamental questions. This is no tongue lashing by any means, but where can you go and get your automobile worked on and not see the shop's rules, the company's policies and the insurance and warranty completely in plain view ??
Did you ever show up at the grocery store, go shopping and walk right by the register with your cart of food without paying ?? I think someone will call you back and have you read the company policy. How 'bout going to a baseball or football game. The rules are so big in writing you have to be a complete moron to break the rules.
Why are you different and expect people to obey your thoughts ??