Yeah, ain't making double flare brake lines fun? Not! Anyhow, with all the above suggestions I have another that's helped me over the years = a little brake fluid where the cone on the tool meets the line/area to be flared, sort of just helps a bit if for some reason the cone tries to turn as you're crimping, allows it to turn with the crimp versus not turning and putting a flat crimp in the double flare, which of course means "cut it off and start over".
