You might ask the vendor or designer to recommend someone local to you. In the case of my ring, it is a designer/''brand'' piece that was sold to me through an online vendor, so I located one of the designer''s authorized retailers in my area and have used that for service. Yet another option is to determine who the most reputable platinumsmith in your area is (most likely by speaking to a hopefully unbiased party like a local appraiser) and use them.
Shipping it back and forth is really not a good option - it (obviously) means being without your ring for a longer period; there''s a risk, however slight, of the package being lost/stolen in transit, though if you ship registered mail it really is quite safe, but pricey, and depending how often you want your ring polished the cost will really add up; and it simply is unnecessary. Save yourself the hassle and do what most PSers do: develop a relationship with a local jeweler.