Since most artists are whiny, egotistical shells of a human being, there IS no try and proven technique- other than at LEAST paying them promptly after they have created the art you wanted! Even IF they took longer than you wanted. Even IF they whined about any changes. (Not that I would ever do that.....)
      And, please don't take offense....but maybe even throwing a tiny, little compliment about the art would help. I know- I know. But facts are facts. I could whine for months about what I myself have had to endure with some of my clients- but I can't find my hanky.
      So, there you HAVE it! If you pay promptly, and Heavewn Forbid, even toss a microscoptic compliment their way, you are  in a MUCH better position to actually having a Good Relatioinship with your artist!