I use a piece of smooth leather (an old piece of belt works great) as a strop and put the tiny tiniest dab of brass polish on it (for both brass and SS nozzles). A piece of old denim works great too - I even use the jeans I'm wearing if I can't find the strop!
Lay the leather on a hard surface and briskly and lightly rub the tip of the nozzle on it. I use a circular rocking motion so as to "polish" a slight curvature to the very tip of the nozzle (the E3D nozzles work great with this). See the attached photo.
Once polished, I squirt isopropyl alcohol from INSIDE the nozzle to force out gunk - but more importantly, to watch the stream of alcohol to insure that it is smooth and has even flow. If not, that indicates a clog or, most likely, a burr at the tip of the bore. You can use a wire or the shaft end of a micro drill like these to reshape the opening. Run the wire or drill shaft down from inside the nozzle pushing out the tip. This will reshape the opening to be round and remove any burrs. Finish with a quick 1 or 2 stroke light swipe on clean leather (no polish) and you'll have a bright shiny polished nozzle tip with a good round opening like this one:
Notice that the edges around the flat area around the orifice is very slightly rounded and the edges are rounded over slightly. You'll find your first layers will go down much more smoothly and your top layers will look much nicer.
After the initial polishing, you can touch up the nozzle with it installed in the hot end. Use a leather strop without polish and just take a swipe or two across the nozzle tip.
To test, I created a simple "single layer test" file. It's simply a 75mm cylinder .2mm tall (representing a very common single layer height). It is a great object to test and refine your first layers and test the effect of polishing.