Start with OFP now & get that little gil moving I say. It cat help to get into practice with all the positions to move her vertx & anterior (head down, back to the front). Check out spinningbabies.com - Home for some great ideas on how to get her into the right spot. One really important thing to do IMO, is to not recline back when you're sitting. Lean forward on the lounge with cushions behind you, sit on a fit ball, have you carseat upright. It makes a huge difference, my posterior baby turned in a week of me consciously remembering to not recline

Having said that though, 12 weeks is a long time to get her to move so I wouldn't be stressing too much about it just yet