I think AT is dead on here. Don't take the response you anticipate into account (predicting a response is something that should be avoided anyway). Make sure you communicate your boundaries assertively and consistently.
Don't allow her to cross your boundaries a few times before pointing it out, do it as soon as it happens. If you start to have an emotional reaction to something she says, use this as a sign that something isn't right, and try and clarify whether a boundary has in fact been crossed or if you are reacting emotionally for another reason.
In the example you gave it seems to me that her initial question was an attack. Was your response a parry, or were you really seeking clarification of what she meant? If you felt attacked you should have stated this immediately.
I get the sense that your W felt disrespected by you leaving the magazines in the house, so she came after you with the intention of countering your "attack." Maybe this isn't the sort of situation where a boundary needs to be enforced. Maybe you could have acknowledged that she was upset, and tried to validate her feelings. Underneath it all is a woman who feels mistreated because her space wasn't respected. Had you acknowledged this, you might have made a connection instead of heaping more pain onto the pile.
I think the key to this stuff really is being assertive. Learn when exactly your boundaries are being crossed and the enforce them. The ultimate goal of boundary enforcement is not to shut the other person down, but to redirect both of your energy into a common channel, so that you both exist comfortably side by side. If you look at things this way, you both have energy that moves in a certain way, validation and boundary enforcement can be tools to acheive this goal in different situations.
“No problem can be solved from the same level of consciousness that created it. ” – Albert Einstein
