Even for the best, most experienced folks, the trimjob is never quite *perfect* - there will always be little tidbits somewhere on it that are awkward to get at, or could be shaped a little better, or might not cook at the same rate, etc., etc. The hope is just to get far enough into diminishing returns that cutting/leaving them won't make any discernible difference in the end result, anyway (threshold may vary, based on the cow, the audience, the weather, and your own energy levels).
But yes, practice and a good knife will make trimming go faster and give you confidence in knowing what/how much to remove and what to leave. Pay attention to the brisket after it gets cooked; the bark and the meat will clue you in to places that should have been trimmed more/less.
