A green stick off a tree works perfectly fine but, how about the buggy whip if you want to get fancy? Yes, it was still a Rule of Thumb that we broke horses to both ride and drive but by gosh there really was a school bus to carry us to school. The driving part was purely for pleasure - honest
If you want to get fancy, tie a live carrot to the end of the buggy whip
This is where I could really get on my negative NH soapbox --- when it sadly comes down to where someone thinks a horse can't be trained unless they've got a magic carrot stick that costs a lot of money. How much ARE those things anyway?