I have imported guns from Switzerland and am about to import from Germany. Here is a current summary of my experience, linked to relevant online resources:
Research comparable values. Study all relevant web pages in the foreign top level domain of your concern. In my case, an interest in the SIG P210 calls for all Kessler catalogs and price lists. Here is a search query composed in accordance with my interests. Bear in mind that all long-distance purchases involve a risk. With market prices abroad on items of my interest running between a quarter and a half of market prices for comparable items stateside, my risks are well justified. Likewise in cases when such items cannot be had locally for love or money.
Find an export agent. Your best bet for finding an agent willing and able to handle your firearms lot for export from the foreign country is online auctions. For example, the Swiss dealers selling on Gunbroker include AfA and swissdagger. Make sure that your export agent understands the legal requirements for shipping firearms to the U.S. Also make sure that he has the right connections to do so. For example, Swiss law no longer allows shipping firearms by mail. Many common carriers follow suit, refusing to accept firearm shipments, unless the sender cultivates a "special relationship" with them.
Choose wisely. Generally you will have to pay for your firearms before you can apply for export and import licenses. The firearms you import must be deemed suitable for "sporting use" and attested as having remained in the country from which you are exporting them for the past five years. Only civilian firearms and foreign military firearms that qualify as curios and relics can be imported. U.S. military firearms cannot be re-imported.
Select a U.S.-based importer. I am paying through the nose for import licenses and international courier services, but Andrew Zink (AfA) and Stefan Mahrer (swissdagger) have access to common carriers and less costly importers. Make sure that the importer that your export agents recommend will mark your gun discreetly, e.g. inside the magazine well or under the stocks.
Stay legal. ATF requires licensure of both the importer and the import itself via the ATF form 6 application. Of special importance on this application are items 19 through 24, which discuss release of the firearms shipment from Customs custody. Also of importance is the form 6A, which must be presented to Customs at the time of its entry. ATF Form 6 s only good for occasional private imports via an FFL holder, for your personal use, but no one will stop you from reselling some of your personally imported guns after a while.
Consider using a broker. A licensed customs broker located at the port of entry will be able to submit the license and release documents locally. National Customs Brokers Association lists local associations of individual brokers. Port of entry information is available from U.S. Customs. Import brokers charge a fee for a Customs entry, plus charges for messenger services where applicable. Brokers may also charge a fee for government agency submissions, its amount depending on the complexity of the agency requirements. Additionally, a customs bond will be required, either as a continuous bond for ongoing imports over a calendar year or as single entry bonds per each instance of importation.
Good luck. Please feel free to pose further questions and requests via email or phone 323-363-1860.