1) The agency
Every man and his dog in northern Thailand runs a trekking company. Some years ago the TAT tried to clean up the industry with some success, but a few shonky operators still lurk around. Just because your Chiang Mai guesthouse says they organise trekking doesn't mean that they organise trekking -- it often means they are selling you a place on a trek that is run by a separate company for which they'll get a cut. Grilling a guesthouse manager about a trek that they have no control over is a waste of time. Go to the source when shopping around -- ask "Do you run your own tours?" and if they say no, go elsewhere. If it means the guesthouse throws you out because you're not trekking with them, they obviously don't deserve your money anyway. Does the agency appear to be professional? Can you see comments from past groups? Can you talk to a returning group? How does the agency treat you? Are they corporate or family run? Both can be good. A family-run business will often have a more down-to-earth feeling, but this can easily become shoddy.
2) The guide
While the guesthouses are often middlemen, the guides are often freelance, meaning that just because your mate Jack did a trek with XYZ tours and the guide, Lek, was excellent, doesn't mean that Lek will be leading the XYZ tour you book yourself on. Try to meet the guide beforehand. If the agency is evasive, go elsewhere. The guide will play an important part in determining the type of trek it will be -- are you more interested in guzzling home-made wine, smoking pot around the campfire and snogging a fellow trekker, or are you more keen on learning about the ethnic makeup and power relationships within a tribe. Your guide should be able to provide you with the experience you are after? (And many guides will quite probably be happy to snog you.) Meet him or her first.
3) The group
The group can make or break the trek and thankfully most guesthouses will show you a list of who is on a trek beforehand so you can have a rough idea of what you are getting yourself in for. Sex, age, nationality are all normally listed. If possible try to meet beforehand, but often that isn't feasible, so you'll just have to take an educated guess about whether you'll be hooking up with like-minded souls. If you can meet beforehand, find out more regarding everyone's expectations -- why are they trekking? Why are you trekking? There is little point keeping secrets as by the end of five days together, you'll know more than you probably want to know about them all (and they about you). If the group isn't to your liking, drop out and go find another trip.
4) Your expectations
Why are you trekking? If it's to see remote ethnic groups who have never seen a foreigner before, stay at home and buy a copy of National Geographic. If you are doing an organised trek anywhere in Thailand, rest assured you and your group will be very familiar to the people you visit and be prepared for a touristic experience. High expectations of an authentic experience with no commercial pressure to buy anything are likely to lead to severe disappointment. You have paid money to be taken on the trip -- a commercial enterprise in itself -- so don't find yourself resenting the fact that villagers hope to make some money out of you. If you are uncomfortable in the voyeur seat, then trekking is most likely not for you -- perhaps do a trip that involves just walking and caving but none of the "canned" hilltribe experiences. Nan is a great option for treks of this kind.
5) The trip
This is often a case of less is more. Seeing five villages in a day is neither comfortable, enjoyable or particularly interesting. One or at most two tribes a day is more than enough. Find out how long you will spend in a village if you visit during the day. Will you be able to participate in village life in anyway, such as by planting rice, changing a roof or gathering wood?
You're on a "trek" after all and a fair amount of walking is a good thing, though you don't want to be on your feet ten hours' a day. Go through the trip in detail before signing up -- how much walking is there each day? Are there long breaks? What is the terrain like? Will you be mostly walking through rice fields, jungle or along a main road? How much driving is required to get to where the trek starts? If you're driving for more than a couple of hours to get to the starting point, consider starting the trek from further afield. How much food is supplied? Will you be supplied with as much water as you need? Do you have special food requirements? If you do, be sure to say so. What villages will you be staying in? Which tribes will you be visiting? What pace will the trek walk at?
6) Your fitness
The treks are generally moderately strenuous, though this does vary. You should be of at least an average fitness level. A good pair of boots is a good idea, though people continue to go trekking in flip flops. Take the bare essentials and use the safety storage at your guesthouse for the bulk of your stuff -- there is really little need to take a laptop with you. Trekking in Thailand generally doesn't involve porters, so you will need to carry everything you take with you. Take sunscreen. In cool season, take warm clothes and make sure warm bedding will be available in the evening -- warm bedding is often better than sleeping bags as the latter can get very crusty and yucky if not cared for.
7) The extras
Rafting and elephant riding are the two most common add-ons to a trek. The novelty of riding an elephant wears off after about 3 nanoseconds, and the rafting is more like floating -- don't envision a http://www.bbxrafting.com/pages/destinations/nepal/sunkosi here. A trek without elephant riding and rafting is also cheaper.
Remember you're in Thailand to have fun. Treks, like many activities in Thailand can be a lot of fun, just remember to pack your sense of humour along with the sunscreen and if you do your research before signing up, chances are you'll have a lot of fun and a great trek.
Stuart McDonald co-founded Travelfish.org with Samantha Brown in 2004. He has lived in Thailand, Cambodia and Indonesia, where he worked as an under-paid, under-skilled language teacher, an embassy staffer, a newspaper web-site developer, freelancing and various other stuff. His favourite read is The Art of Travel by Alain de Botton.