Girlfriend vs. Sub

Ok, I'm finding myself with a bit of a dilemma. I have a hard time figuring out when to switch from girlfriend to submissive. We get along great and sometimes we'll be sitting there joking and it's hard to know when to start with the yes sir stuff. I think this is even more true if things have been stressful or I'm in a situation I might find a little intimidating.

It's been kinda interesting, as we've become more involved in local groups, I've had a chance to observe different dom/sub relationships. Some friends of ours are in a mistress/slave relationship, but it's hard to draw too many correlations there, because she is married to a bdsm-friendly man and her slave lives with them. So they always know how their relationship is defined. I'd like to talk to some married dom/sub couples and see how they deal with this issue. We may just have to come up with some kind of signal, at least until I figure all this stuff out and can tell when I need to be his submissive vs. his girlfriend.