I travel with a copy of the Air Carrier Access Act as well as a print out from the airlines own website when I fly. I've had about a 90% success rate requesting and having my manual wheelchair placed in the cabin closet by doing that. Generally, the flight attendants don't believe me that it is both the law and their airlines policy until they read both of those copies themselves.
It has been twice that I've needed to request a Complaint Resolution Officer and in both cases, that person did tell the flight attendants to unload the luggage they had placed in the cabin closet to allow my spouse to attempt to place my manual wheelchair into it. The wheelchair has always fit and I have always received apologies from the Complaint Resolution Officers and a request to send through a complaint so that they can better train their cabin crews.
Before I did this, I've had my frame bent, spokes broken (not easy on a manual wheelchair) and even had the frame broken. They've also forgotten my wheelchair in the jetway where it was placed with the gate check tag and then failed to notice it and load it into the hold of the aircraft.
I am always friendly, upbeat and positive. Mostly because that is just my normal personality.
I have found that being that way helps a bit but what helps more is acknowledging to the cabin crew that it IS a pain for them to have to deal with a manual wheelchair and apologizing for the extra work.
Although now all of this is pretty much a moot point, as my current collapsible manual travel wheelchair comes apart and folds down far enough to fit into the overhead bins.
The last time we flew, we had to call a Complaint Resolution Officer to require the cabin crew to allow us to place my manual wheelchair into the overhead compartment because they didn't believe us that it would fit. It did, they were amazed and apologetic.
So, when we flew home, my spouse just put it up in the overhead compartment while I was being pre-boarded in the aisle chair. That particular flight crew (very decent, kind group of people) made it a point to tell us to not even bother to ask any cabin crews about placing it in the overhead compartments. That the best thing to do, was just put it up there and tell them after, as it would save us a lot of hassles with no one believing it will fit up there. That's the plan for the next flights we take.
Oh, and when the wheelchair is up in the overhead compartment, it fits nicely on top of our carry-on bags and doesn't take up any extra space.