Having done this recently, I can tell you I used Ted L. Rausch to handle the customs side of things. The decision to use them was based purely on convenience; their location was fairly close to me, and they have established connections with the port and US Customs and Border Protection. I'm not sure whether I could have made my way through that myself as they needed to file paperwork in the USCBP system before the vessel sailed, and then again just before it arrived. They did this through an on-line system linked to USCBP and I don't know whether you can get access to that as a private individual.
Depending on exactly what it is you need they may, or may not be the best choice. My import was fairly simple - a pre '68 car - so there wasn't anything else that I couldn't do myself. I completed all my DMV, EPA, and DOT paperwork (including the stuff I sent to Rausch for customs clearance), and handled VIN verification and title application at the DMV myself. If you need someone to do this for you, or if your import is something which needs DOT and emissions certification then I don't think they'd necessarily be the best choice.
Are you able to provide more information about what it is you're importing? That may help with the recommendations.
Check this thread for some additional information:
http://forums.pelicanparts.com/off-topic-discussions/932339-importing-car-usa-overseas.html