I usually assume that I'm angry about something that came up while my therapist was sitting there saying something not at all anger-provoking, so I try to work out what it was that set me off. If I need to talk to her about it, I talk about how X made me feel, not how she made me feel. I guess that's using "I" statements, but I don't really think of it that way. It's more a matter of trying to get to the bottom of what upset me. My therapist is just not the type to be insensitive or hurtful, so if I feel myself reacting to something she's said or done, I have to ask what that thing is underneath. It saves on apologies later.