wish I could help on that, but have no tips whatsoever! Our two mares (despite being very similar in build/type/age/workload/rugging) grow their coats back in very different time frames!
I know some people swear by keeping their horses that bit warmer, but I hate them any warmer than essential so that isn't something I try. I'm not convinced that it would be totally sucessful anyway as it is the length of the days that governs coat growth aswell as temperature.
If you only want to clip once in the winter, clip as late as possible, rug early and well so they dont have a thick coat too soon, and once theyve been clipped (we clip around novemeber) rug up well and should last you all season!
echo clipping as late as possible, the days getting shorter and darker also triggers the hair growth, so if you clip while the nights are still drawing in you find that first clip grows out literally within a couple of weeks and you have to clip again but if you can hold off for as long you can avoid needing this second clip so soon.
I usually do my first clip after my week away in Oct I always go away the first week and have to try my upmost not to clip before even though I really want to but if he's having a week off there's no point.
The 1st clip always grows back really quickly, I clip again then when its growing back, I just clip when I want, sometimes 4 times, I have my own set so dont' have to pay anyone to do it which is a bonus
So if you are having to pay someone and only want to get your horse clipped once or twice I'd echo what others have said, keep warm and clip as late as you can