This is a question that is highly debated in dentistry; there are strong arguments for both methods. It is the long understood method to brush then floss and dentists have long taught it this way.
Recently though alternative arguments have arisen for flossing before brushing. Flossing before brushing allows for fluoride from the toothpaste to reach between the teeth when brushing as plaque has already been removed which is beneficial in strengthening the teeth and preventing cavities. Furthermore, when you floss, some plaque debris can fall onto the teeth and if you have previously brushed your teeth then this plaque will linger. However, this can be avoided by rinsing your mouth out after flossing with water or mouthwash as recommended.
There really is no general agreement amongst dentists that one method is superior to the other. The only thing that is agreed upon in this argument is that both flossing and brushing should be practised daily, regardless of the order.