Note that the airport you may see marked on some maps in Mae Sariang is a private aerodrome, so only useful if you have your own plane.
Night sleeper buses run between Bangkok (Mor Chit) and Mae Hong Son daily. These buses will pass through Mae Sariang and Chiang Mai on their way to or from Bangkok. These air-con and VIP buses do then leave for Mae Hong Son at quite odd hours (because these are night buses arriving from Bangkok) so unless you have an important breakfast date in Mae Hong Son, it's better to forget the VIP buses and take a slower fan or standard air-con bus leaving at a more reasonable time -- even if the scheduling gets a bit sketchy for these local buses.
Buses to and from Chiang Mai run through the day and into the evening and take four to five hours stopping at Hot, Chom Thong, Sanpatong and all points east to Chiang Mai. Alternatively you could take a faster minibus which also leaves from Mae Sariang bus station but has far fewer stops on the way. Note that minibuses tend to leave when, and only when, they are full.
Mae Hong Son is also around four hours, stopping at Mae Lanoi and Khun Yuam.
Finally, you will also find songthaews hanging around the bus station and opposite the market, offering cheaper, slower, less comfortable, but sometimes more fun rides in a meet the locals, sit on a chicken, have a Karen baby vomit all over you sort of way. These leave sporadically for local destinations such as Khun Yuam, Mae Sam Laep, Hot and the mountain villages south of Mae Sariang. Note in a small, remote town like Mae Sariang all times and routes are going to be highly changeable so please double check before turning up at the bus station.
Sample bus departure information:
Bangkok: Sombath Tour run services that depart at 17:00 and 19:30 taking approximately 12 hours and costing 580 baht for the standard sleeper and 680 baht for the later 1st class service. Both terminate at Bangkok’s Mor Chit terminal. These are coming from Mae Hong Son so may or may not be full if you haven’t booked in advance. If you can find a seat on one of these air-con coaches then they’ll charge you 180 baht for Chiang Mai. Check times and buy tickets in advance.
Chiang Mai (fan): 12:30, 00:00, 01:00, 5 hours, 100 baht.
Chiang Mai (minibus): From 07:00 onwards, hourly or whenever they are full, 3.5 hours, 200 baht.
Mae Hong Son (fan): 12:30, 00:30, 01:00, takes 5 hours, 100 baht.
Mae Hong Son (minibus): From 07:00 onwards, approximately hourly or when they are full, 200 baht.
Khun Yuam (minibus): 10:30, 14:30, 16:30, 17:30, 2.5 hours, 150 baht.
Mae Sot (minibus): From 07:00 onwards, leaving when full, 200 baht.
Mae Sam Laep (songthaew): Hourly during daylight hours from in front of the day market, 70 baht.
Mae Sariang Bus station: Mae Sariang Rd; (097) 951 1253.
You'll have to rely on motorbikes or bicycles to get around Mae Sariang if you don't want to mess around with songthaews. There's certainly more than enough interesting destinations and scenic routes to be taken in the vicinity of Mae Sariang to make hiring a bicycle or a motorbike a good option.
Honda Dreams or Waves are available from North West Guesthouse, Riverhouse Resort and Riverhouse Hotel, usually for 200 baht per day, though they don't have a lot. Other guesthouses may be able to rustle you up one. At time of writing there are no dedicated hire shops in town.
North West also offer regular or mountain bikes for 50 and 100 baht per day respectively and Riverbank had a few bicycles for guest use. Most hotels and guesthouses are also more than willing to find you a car or pick-up plus driver for the day if you just want to cruise around rather than opt for one of their set day tours. Car hire cost will depend upon distance and destination.