Writing letters, going to small claims court, pursuing arbitration or hiring an attorney are the typical ways to enforce your rights. There are a few other options in specific circumstances.
Customs can be one of your most powerful allies against copyright and trademark infringers. For a modest fee, you are allowed to “record” for twenty years your registered copyrights and trademarks with Customs, which then blocks the import of any infringing goods if they are discovered. You can familiarize yourself with the process by asking Customs for Customs Publication 563. To make sure that the information is current, contact the Commissioner of Customs through the above link.
Infringers HATE when artists register. Imagine if you notify Customs that some company is importing without authorization your design on an t-shirt. Customs, in its discretion, will then search shipments it deems likely to carry the t-shirt. Even if the importer only brought in one batch of your t-shirt, chances are that much of its business involves using stolen artwork and so the infringer is likely to be hyper-sensitive about Customs snooping around all of its shipments. If the infringer is run by prudent businessmen, you will have the opportunity to negotiate a settlement. If not, you at least have the chance to reduce the number of infringing goods that get through. On the other hand, do not threaten or actually block an importer unless you are sure that they have imported infringing work in the past. There is a often a lot of money at stake, and you don’t want to hurt an innocent importer who can then sue you. Even if you are correct, it is best to consult an attorney early in the process since any well funded importer is likely to do so (and, if it is an infringer, it probably has no morals and will fight you in any way it can.)
The regulations guiding Customs are published at 19 CFR part 133—Trademarks, Trade Names, and Copyrights. Since the regulations are republished annually in order to include modifications, a stable direct link is impossible. One option is go to the Code of Federal Regulations page, click on the most recent version of Title 19, and search for the terms Trademarks, Trade Names, and Copyrights. The second or third document in the list of hits should have this title.