I don't have any real advice as to what worked for us, but my son did eventually outgrow this stage. Once he turned four he just decided one day that he didn't like his hair in his eyes. I said, "Well, if you would let the girls cut it at the salon, then your hair wouldn't be in your face." He agreed that he was ready to get his hair cut. Of course, this was the Wednesday before Thanksgiving, so finding a salon that wasn't packed was an issue. We ran into Supercuts and he sat still the ENTIRE time and never once cried. He has been a perfect patron ever since.
We, too, had to cut his every month,so it was always a challenge. I would try to stay calm and relaxed myself and that sometimes helped. We also would give him a sucker while he sat, which sometimes helped, but not everytime.
I do have to say, however, that the girls at Lollilocks on ValVista and Baseline, were the most patient and kind girls that we ever had. He squirmed and fought the entire time and still got a wonderful haircut. It is really a salon geared more for little girls, but they do have a car, an elephant and one other novelty chair for the children to sit in. I hope this helps. I guess the number one thing to remember is that this too shall pass:)