I helped a friend win last year.
I made a spreadsheet with all of the homes in their neighborhood. It included the address, sq. footage, appraised value - current and past year, increase in appraisal - and the 2 or 3 decreases, % appraisal increase, $/sq. ft.
I showed that their house was valued near the top per sq. ft. in the neighborhood and was out of line with comparable homes.
I also had photos of the "deficiencies" in their home and estimates to bring them up to good.
I could have presented more than 30 minutes analyzing why their appraisal was too high, the board surrendered after just a few minutes. One commented, "Boy, you were prepared!"
I did meet another couple while waiting, they said they protested and lost every year. They brought nothing with them other than saying that the appraisal was too high. If you try that approach, unless your appraisal is absolutely crazy, you're not going to win.