One option you have is to hire a motorcycle taxi, they have the expertize to learn you the in and outs of riding a small sized motorcycle. They often also speak some degree of English, so communication can go a bit smoother.
And last, they have most likely the motorcycle (in size) which you want to ride later, so tricks learned will be actually helpful.
Hiring a motorcycle-taxi for a day or part of the day would maybe not be the cheapest, but it is surely an alternative... The cheapest option would be somebody living in Chiang Mai who can show you the tricks, and would accept a few beer (after the training) as compensation.
- 
automatic motorbike lessons in chiang mai?
