I have been living in Bangkok for a year and am planning to move to Chiang Mai. I don't have a lot of stuff, but I do have international baggage allowance plus a few more things I've accumulated over the last year.

At first I was thinking about flying, but I don't think it will work with the amount of stuff I want to take with me. Does anyone know what baggage allowances are like (and how they are enforced or not) on the train and buses?

Also, I have heard rumors of theft on the train and poor driving on some buses and would appreciate any collective wisdom you can share.

The best bus service from Bangkok to Chiang Mai is Nakhon Chai Air. They keep the speed limit (GPS-controlled, I believe), are mostly on time and their busses are very comfortable, the best ones (First Class if I remember correctly) being 3 abreast with business class style seats and privat TV http://www.nca.co.th/index_en.php

They have their own bus terminal not far from Mo Chit and the best option is to book in advance and take a taxi directly to the terminal. Tickets can be booked on telephone and paid at 7-11, but sometimes it's difficult to get through on their phone lines. As for luggage allowances, they are rather generous, and in any case it's possible to take extra luggage paying a fee.

Be aware that taxis at Mo Chit BTS station for some reason refuses to go to the NCA bus terminal, or at least, that's what I've experience in the past.

Weigh your stuff, then check out the rate at your local Thai post office. FYI, I see a lot of motorcycles snail mailed via my PO, the guys there tell me the run about baht 1,000+ for postage, not too bad.

Mac

why not just hire a van,that way you can go with your stuff,I'm sure you can find a van + driver in Bangkok

Pack up whatever you need to shift and head to your favourite post office, you'll be amazed how cheap it is. Reason I know is a mate is moving from Chiang Mai to the back of beyond of Korat and has been going through the same as you and he's elated at costs.