All tours begin and end in Chiang Mai, Northern Thailand the motorbike capital of the country. Getting to Chiang Mai is very easy and just a 1 hour flight from Bangkok and flights are very low cost.
Tip: There are 2 airports in Bangkok, 1 serving international flights and regular priced domestic flights and another that serves only low cost airlines. Both airports are quite a distance from each other (Up to 1 Hour).
If you are landing in Bangkok International Airport (Suvarnabhumi) and plan on joining a connecting flight to Chiang Mai on arrival book a flight with Thai Airways & Bangkok Airways Their flights are not expensive and they fly out of the main airport plus they allow a good baggage allowance!
If you are on a budget and/or you are staying in Bangkok before you come to Chiang Mai try Lion Air. Flights with them are very cheap especially booked in advance but they fly from the second airport Don Muang.
Very BIG tip: If you are catching a taxi in Bangkok to any of the airports MAKE SURE you tell the driver the name of the airport and make sure he fully understands you. Being taken to the wrong airport is not uncommon.
Trains, Buses? Forget it!