If you follow rule #2, most of these other rules will fall into place. A friend of mine got engaged at the same time I did last Christmas. His initial stone was larger than the one I got but the imperfections were obvious even to a novice like me and he went to Jared (no kidding). He overpaid in a big way and I believe he regrets it to this day.
My jeweler worked with me to find the perfect fit in terms of price and size. The entire staff was willing to help me out and in the end they were spot on in terms of what they suggested. Take the time and maybe pay a little more to go to someone you trust.