DAX futures - when to average up on a position?

Still getting my feet wet with DAX futures, and I'm finding myself in a bind. I'll open a position, say long, it moves against me a bit, so I add more, lowering my average cost. Sometimes it pays off, sometimes I just double down on a bad call. For those trading $FDAX, when do you decide to average up, not just down, on a winning position? Or do you just let it ride with the initial entry?