A basal thermometer will only tell you when you have already ovulated. Ovulation predictor kits will tell you when you are fertile and should DTD. I used a ferning microscope (a little one, looked like a lipstick thing) for trying-to-avoid, in combination with basal temp tracking (I was worried it wouldn't be accurate because of night-nursing, but I didn't seem to have an effect on my basal temp reading)... the thing worked like crap. I don't know if it was just me, I'm pretty sure I was using it right. But I would get ferning when I wasn't ovulating, and none when I did...
