A kinder, gentler term for 'ex' that I catch myself using every now and again. It keeps the rancor and bottled-up bitterness at bay. It's appropriate for those break-ups that, while they don't necessarily involve thrown objects and restraining orders, still are cause enough for hard feelings and unanswered questions.
That, and the term 'ex' sounds kind of pathetic, in my not-so-humble opinion. It defines yourself in terms of the relationships you don't have, rather than the relationships you once had. 'Then-girlfriend' reminds the listener that once upon a time, you snuggled up to her on rainy Sunday mornings and promised her your undying devotion. It emphasizes that once upon a time, she was something more than awkward, turned-away glances and tears in the pillowcase. Time heals all wounds, and euphemism really does help with hard-to-deal-with feelings.