In addition to this, which I wholeheartedly agree with, the tactic which is their most powerful, is often unpopular. The Strike.
The Southern California Retail Clerks in 2003-2004 is instructive to the dangers of a strike.
So many hard feelings were created between customers and store employees, the supermarkets ended up transferring employees from one store to another at the conclusion of the strike.
One day your friendly cashier is asking you if you found everything you need, then the next week she is aggressively haranguing you as you cross the picket line to buy diapers for your kid. When the strike was over, the demeanor was far less friendly and seemed rather insincere.
The store employees would also bash the store, telling tales of what a crappy company it was, and how management couldn't care less about the employees and the customer, you. After the strike, I'm supposed to forget about that?
Lastly, it has unintended consequences, such as shifting long term shopping patterns.
In my case, I had lived in the neighborhood for about 3 years at the time of the strike, and did 90% of my shopping at the Vons less than a mile from my house. During the strike, I avoided shopping there, not so much as to support the unions, but to support the people who I had grown to develop at least a friendly, casual, relationship. Although I did cross their picket lines a few times during the 4 months, like getting diapers or flu medicine at 11:00 at night.
However, I did A LOT more shopping at Costco, Trader Joe's, and Sam's Club. I also shopped at an Albertson's about 2 miles away that was on strike as well, but I had rarely shopped there before, and hence did not have the feelings of guilt crossing their picket line as I did the Vons. The upshot is, I found the that the other markets had either better quality, better selection, better prices or a combination of all three than my local Vons. I now do about 10% of my shopping there.