I first did this when I was teaching a group therapy class, and it's become a standby technique in the seminars I've run since. If you're not planning on attending any personal-development programs, you might work up the nerve to ask for feedback from a coworker, a few acquaintances, a 12-step group (obviously, people with whom you feel unusually safe). Explain your boundary issue first, then ask for input. Something like "I keep having the same kind of argument with different people [or dating the same kind of loser, etc.]. Do you see anything I'm doing that's contributing to this dynamic?"
Humans are astonishingly attuned to interpreting one another's social energy, and you'll likely end up with a pretty clear consensus. "You always look down and mumble when I talk to you," they might say. "I feel like you're not interested." Or: "You're so helpful and polite, even to awful people. Frankly, you're kind of a doormat."
One very important caveat: Do not, I repeat, do not rely on feedback from your nearest and dearest. These people are preselected to match your boundary-setting patterns. Your dysfunctions will be as invisible to them as to you. For instance, if a pleaser asks her boyfriend whether he thinks it's normal that she buys him silk sheets, while she herself sleeps on the floor so as not to disturb him, he'll respond that she's the healthiest, most normal person he's ever met. She'd get more helpful data from her dentist.
The Problem with Finding a New Tune
Dysfunctional relationships are rigid. Each person plays one role, and any attempt to behave differently is met by indignation or even aggression. This rigidity (which underlies "isms" like racism, classism, and fundamentalism) makes boundary setting delightfully simple: Folks who behave in prescribed ways are the Good Guys; everyone else is the Enemy. Of course, even a minor deviation can turn friend to foe, so to keep the party going, participants better repress all individualism. To many people, that seems a small price for the intimacy created by a black-and-white worldview. Unfortunately, since this closeness requires self-abandonment, it actually isn't intimacy at all.
Functional relationship skills begin with the realization that intimacy is rarely built upon stiff rules, that most things can't be reduced to black or white. Painters sometimes use a gray scale, a strip of paper that has white at one end and black at the other, with, for example, five gradually deepening shades of gray in between. They hold up this strip to see which value of gray best matches the shade of the color they're painting. Each object gets a gray scale value. (Pure white has a value of 1; medium gray, 4; jet black, 7.) Learning to evaluate levels of intimacy in a similar way is the first lesson of the healthy interpersonal waltz.