Go to you local track and find the fast guys, ask if they don't mind pacing you for a couple of laps, this will give you a front row ticket to how the right lines are found. That is what I do , if I fall off the back I just wait til they come back arouand and hop in line. Most are going to take soem practice laps anyway, that is your chance to learn the line.
Other than that you hit it on the head, practice, practice, practice. I know it sounds cliche' but if you show up early and stay late you will refine your line to the point you can't get it much faster.