First of all, most civil liberties protest actions are carried out by a very small number of people. Only a handful of people participated in the Boston Tea Party. And it was Rosa Parks and a few others, not 90%+ of the population, who refused to sit at the back of the bus. (not trying to compare TSA to civil rights, but making the point about small numbers of people; please don't revive that tired argument.) Every act of resistance to TSA is important, but we can't expect every supporter to resist 100% of the time.
As for my personal limits, here goes:
First of all, I mentally consider before each trip to the airport the possibility that I may not fly that day because of TSA. I evaluate how inconvenienced I would be, what alternatives exist (fly another day, fly another airport, train, drive, etc.). I try hard not to book the last possible flight to make it to an event I need to attend (also helps with other mundane delays/cancellations). I think every person should consider their limits on a trip-by-trip basis. This is the most important aspect of resistance to me, because TSA's power tripping and bullying is based on passengers feeling they must make their flight at all costs.
I will not go through backscatter x-ray unless en-route to the sick/death-bed of a family member, period. I have no intention of going through any NoS in the US, ever, and will not accept a screener's claim that you cannot opt out even if it means missing a flight. I haven't had to consider the issues of a mandatory-NoS airpot (LHR) yet, but I think I would prepare myself to take the train to France and catch another flight.
If wearing a cast, I would not submit to a castscope, even though TSA claims it is mandatory. That is a high-dose medical x-ray (you can see bones) much worse than the backscatter and still operated by untrained morons without calibration or quality control.
I will not remove bandages, wound dressings, etc.
I will not willingly go into a private room and will stand my ground if at all possible. If coerced and I feel compelled to go, I will insist on at least 2 non-TSA witnesses (law enforcement, airline rep, traveling companion, random stranger). If denied witnesses, I will not go into the room unless physically forced to.
I will not "voluntarily surrender" any of the following: laptop, electronics including custom electronics which I travel with from time to time, cell phone, camera equipment, any digital media, prescription medication, or any other non-prohibited item worth more than about $25.
I will not delete photographs from my camera or phone on the request of anyone. Nobody at the airport, including LEOs, has the authority to delete a photograph. Ever.
Here's an odd one for you: I will not take off my eyeglasses. For various medical reasons, the safety from flying debris that polycarbonate eyeglasses provide is very important for me. I generally do not take my glasses off in public places, and I will not do so in an airport.
I will not provide more than one ID. I uses a passport card for domestic travel. I will not provide another ID if asked. The only way they are getting another ID from me is to rummage through all of my stuff looking for it.
I will not provide my mailing address or phone number to a screener. I will not voluntarily provide my mailing address to a LEO. If threatened with arrest, I will probably cave and provide the address to the LEO, but if the LEO turns the info over to TSA I will file a complaint against that officer for misuse of PII.