I am going to put in my 2 cents here, based on my experiences. I am not a hitting “expert” other than what I have learned watching instructors and hitters over the years.
I think that developing a good swing takes months (actually years) of work. I am not sure going to one hitting clinic will do you much long term good without a dedicated daily (or nearly) approach and help from a coach who really knows what he is talking about.
In my sons case he was the best hitter on his HS team however he did it with a lot of flaws in his swing. He knew it, his coach knew it, but in season was no time to revamp his swing. Once he got through a couple of big tournaments in the early summer he stopped hitting moving baseballs and worked on redoing his swing with a really good hitting coach. Initially he worked 2x per week with the coach and then on his own every night working on stuff he learned. They used video, T’s, and a hit-a-way to work on balance, swing plane, foot and hand positions, ball positions, load, etc. At home most of his work was in the mirror, and sometimes off the T. I think this took him 6-8 weeks.
He then started hitting live balls, and he played in the fall. I video taped some live action and we went back over it with the coach and he is now working weekly with him and still trying to get in his own work…maybe every other day or so…that darn school is getting in the way ;0.
When people see him hit, all say he has a great stroke, but his hitting coach (and me now that I know what to look for) still see flaws. His coach says that it will take him until the season to get rid of them and it will be something that he will always fight, because he has been doing it for so long.
My long-winded point is that to develop a good swing it takes lots of daily work and dedication and a very good coach. I have found that very few REALLY know what they are doing and video is a must IMO.