Slow and steady with these guys is always the way to go and produces the best, end results. If you think you are moving too fast, you probably are.
The best technique for shoulder breaking is to use your other arm as a block. When he starts heading that way take your opposite arm, bend it up and place the free arm elbow in the bend of the arm he's on. You can bend your free arm as needed to prevent him from climbing it or climbing around it. If he stops, great. Reward him. If he persist don't give in. Continue the block and quickly station him somewhere. Then try again. It won't take too many times for him to learn that trying to get on your shoulder doesn't produce the results he wants. Always, always give him the opportunity to do it right. Next time he will try again. It takes time to learn new behaviors. He consistent. The first time you cave and let him on your shoulder you are back at square one. He will see that if he persists long and often enough he gets his way.
I have a no shoulder rule here too and while I trust my 3 girls and there are times that I need 2 hands and offer my shoulder briefly they are so well conditioned that they refuse. They want to be put them down, not on my shoulder. Good luck! And finally patience, you will need every ounce you can find, reach deep!!