I'd add another thing: keep people informed. About everything! Make sure everyone knows what committees exist, what they do, what their time requirements are. Allow people to question how things are done. Set up a suggestion box, and invite people to submit ideas. Let them know that some ideas may take off right away, some may be discarded, some may languish for a while until a particular person (or mix of people) seize it and make it happen. Solicit feedback after events that use a lot of volunteers.
When people feel they have a voice that's being heard, and believe that a PTO is open to new ideas, they tend to get involved more, and eventually they might outnumber the others!
Reputations are very hard to change - it takes time and consistent positive behavior. Keep trying to recruit new members, especially those new to the school who don't know the rep of the old group.
If you've done everything you've said...my next step would be to ignore them. That will absolutely drive them nutso...but that's what I would do. You'll find folks that want to be a part of this group and that is where your energies should be.
WE ARE A NEW PTO BOARD TRYING OUR BEST TO DISMANTLE THE REPUTATION OF PTO MOMS BEING A CLIQUE. WE ARE GREETED (NOT SO WARMLY) AT EVERY TURN BY FORMER BOARD MEMBERS WHO ARE 'CONTROL-RELIEF CHALLENGED'. DOES ANYONE HAVE SUGGESTIONS OF HOW TO DEAL WITH THEM? WE HAVE ATTEMPTED ADULT CONVERSATION, GETTING THEM INVOLVED AND EVEN BRINGING IN A NON-PARTIAL MEDIATOR. NOTHING SEEMS TO WORK! HELP!!!!! :confused: