there is a bit of a dodgy way of doing it as well
i was in chiang mai one year for three months, and never crossed a border once...
there are lots of travel agents on thanon moon muang, and most of these, for the price of what it costs on a bus journey up to the border, will take your passport up to the border, give the imigration guy a few bt and he will stamp it into and out of the country for them...
they normally have someone who goes up to the border with a few passports regulary... it is specifically for farangs who could not be bothered to go up there themselves....
make sure u get photocopies of your passport before u send it of and a receipt, just incase.....