I think there's a difference between the occasional slip (still a problem with my Mom even after over two years of being out) and someone who is deliberately using the wrong pronouns to make their own little protest about your transition.
I started asking folks to call me "her" when I started presenting full time to them.. Family and friends were first and then at work when I changed my name. Now, if someone slips, I politely ask if there's a reason they just referred to me as "he" or "him." I found that asking for a reason makes them think a bit more about it than just correcting them. It also gives me an idea if they're doing it to be difficult. If that's the case, I squash them like the bug that they are....

Ok, really... If it's family or a friend, I explain how hurtful it is. If it's at work, I don't hesitate going to HR and asking them to correct the situation. I remind them, at work, that using the wrong pronouns in public could out me and put my life in danger.