I find it's easier to pull off if you boost a little before the start line. It's not as fast as doing it at the last possible point, but it gains an extra .1 or so.

Last time I did the track, I lost my ability to do it properly after doing it like 40 laps in a row, and could only manage it every 1 in 6 laps for the rest of the session... maybe I'm not the best to get advice from