If your art is original and you're the author, you have automatically copyright and the right to license the art. I don't know about CafePress, but some websites (like Facebook) have in their Terms and Conditions something like the following:
You grant us a non-exclusive, transferable,
sub-licensable, royalty-free, worldwide license to use any IP content
that you post on or in connection with <service>
Which essentially means you waive all applicable rights to the content.
Check the CafePress T&C for this!
If you still have your rights, find a good license for your product, or make one your own. You could think of things like author attribution, share rights for (non-)commercial goals, adaptation rights, etc.
Also, make sure you can prove that you are the rightful owner and original author of your product. Someone else can always claim it was him who came up with the idea. Things you could do is take a high resolution photo of the product with a newspaper, but it's always safer to deposit your product at a registering service. Note that you'd have to pay for this. A registering service I know of is the [BOIP], but that one is only for the Benelux.
At last, make sure the people at CafePress can see how you have licensed your product. Also make sure they can contact you for further information. The latter isn't needed, but is good practice.
After these steps, nobody may do anything that you don't want them to do (because of the license). Also, everyone knows what they may or may not do, because of the license information at CafePress. Lastly, when someone does do something they're not allowed to and claims they're the rightful owner, you have evidence that they are not.