Most of us are so concerned about planning our trips overseas, what equipment to bring, how to transport film, the right bag, etc., that we don't give much of a thought to how we're going to get BACK into the country.  Goods bought internationally are subject to duties and fees when brought into the U.S., and most of our equipment is manufactured by foreign companies.  You need to be prepared to prove that the thousands of dollars worth of gear you're lugging around were with you when you left the country, or risk problems during your return inspection.

Fortunately, with a little advance planning, this is very easy.  You need a U.S. Customs Form 4457.  They aren't available for download, but obtaining them is free at any Customs office.  Find the nearest U.S. Customs office, they should be in the government section of your phone book, and pack up any equipment you might be planning on taking out of the country.  It isn't a bad idea to bring ANYTHING that might go overseas, even if you aren't taking it on THIS trip.  Bring everything to the Customs office during normal business hours, where you will be presented with the Form 4457 to complete and your serial numbers will be inspected.

The good news is that the forms are reusable for as long as they remain legible, so you only need to do this once for each piece of equipment.  Obviously, if you get something new, you'll need to repeat the process for that piece prior to your next trip.  It's a good idea to keep a copy stuffed in your equipment bag and another one with your passport, just in case.  Be warned that if you have anything repaired while you're out of the country, the repairs or alterations are subject to a duty.

If you have any questions, it's a good idea to call your local Customs office or visit their website at http://www.customs.gov.