This tip goes for indoor and outdoor back lighting but I typically use it more for indoor back lit situations. It's all about the eyes. In my Editing and Consistency class, I give a ton of tips for bringing out a subjects eyes and making them pop because the truth is that, almost no matter what else is happening in an image, your viewer will look at the eyes in the frame first. It's just basic animal instinct. Even your cat won't look at your leg, it will look right into your eyes (I've done extensive research into this by staring at cats). It's just how we connect and we should make that connection, and that first impression as strong as possible.
For those of you who saw the term "catch light" and are wondering what the heck I'm talking about, a catch light is that little glint in a model's eyes. The purpose of a catch light is to create a bright highlight in your subject's eyes which gives them a bit more pop and makes them seem more alive. In back lit situations this is especially crucial because the models entire face is in shade which can leave the eyes looking flat and dull. When I'm shooting outside I add little catch lights by wearing a light or white shirt. It's that simple. The white reflects the sun back into their eyes and creates that little glint. When I shoot indoors with the window light coming from behind my subject, I have a reflector right behind me bouncing light from the window back into her eyes to create the glint. Another killer thing about adding in that little catch light is that it also lightens up the iris of the eye which means you see more of the actual eye color. Heck yes.