I mostly just stuck at bulls up to GM, but you will find the skill gain slows down as you get higher unfortunately.
Your GGS gains will also slow down if you've got a lot of skill points in use on the character you're training. So one way tamers can speed things up is to soulstone any unnecessary skills off temporarily and then train taming while their GGS gains are a little faster.
In honesty once you get around that level it's worth thinking about working the zoo turnins with some jewellery to help you tame unicorns etc, or tame another high end pet repeatedly (eg work cu sidhe to get coloured pups to sell). Then you'll get gains, but you're gaining something else in between, whether it's zoo points or gold from pup taming.
If I don't have much time, I just get a GGS gain from bulls, some days I'll work the bull spawn for over an hour, other days I'm cu taming. I find mixing it up helps a lot, rather than putting on some jewels and taming ridgeys for hours.
Wenchy