That undercurrent of annoyance that you're feeling is not uncommon, which is too bad. You are absolutely correct that parent involvement is essential to your school's success, so giving up or lessening your presence at school is not an option.
That said, I would work to lose the idea that "they're the paid ones and we're the volunteers." They're also the ones that have to be at school 35 hours per week and grade papers at home and prepare lessons and answer to their "boss" and discipline sometimes-wacky kids. They can't come and go as they please. (Just trying to show the other side of the coin.)
If that idea -- the idea of: "they should be thankful we're here; we're not getting paid for this" -- is coming out from your group at all, then that's a part of the issue. By the same token, the teachers and staff should not be giving a "they should realize we're the important ones here" message. Both of those messages contribute to the problem.
The best situation is a realization by both parties that you are 100% on the same side -- you want to create a great place for kids to learn and teachers to teach. That's usually best achieved when the parent group does all it can to recognize that teachers doing great work in the classroom is their most important job and when teachers realize that the efforts of the parent group (this is a proven fact) can help them achieve their core aim of successfully-learning kids.
No how-to here, sorry evol, but more of a philosophy. Don't worry about what "they" won't do for you or that they don't appreciate you. Put a smile on, always stay positive, serve (the kids, the staff, the teachers). Share the research results (available on the involvement pages of this site) subtly and repeatedly with everyone over the long-term. In the end it's far less about being appreciated by the staff and far more about doing great things for the kids. Kill 'em with kindness.
I am fairly new to this type of organization (served 1 1/2yrs), but I have started questioning the effectiveness. I love being an involved parent, and I know our organization does so much for our school and community. I have got an overwhelming sense of burden from the staff. It seems that more often than not they are irritated towards us. From talking to staff and teachers, I have gotten the impression that we bother them. They act like we are in the way and causing more harm than good. I don’t like feeling this way because the school would suffer without our participation. Where do you draw the line? After all, this organization is for OUR children. How do we bridge the gap between staff and volunteers? I think the school forgets that we are not paid employees, we are volunteers.