There are 4 or 5 local buses a day from Chiang Mai Arcade bus station, journey time is approx 4hrs(aircon)/5hrs(fan) and around ~100bt, may drop off at edge of town near the museum. There is also a faster minivan service, departures from Chiang Mai Arcade bus station daily at; 9am, 10am, 12noon, 2pm, 5pm. Journey time is approx 3.5hrs and tickets 200bt.
There are regular orange songthaews from Mae Sot, 5-6 hours.
Coming from Mae Sot (south) just follow Rt 105. It's about a 240 km drive. There are a few gas stations along the way (a big one in Mae Ramat and a brand new one just south of Mae Tan) but it is best to fill your tank before leaving Mae Sot. If you almost run out of fuel, drive into one of the villages on the way and ask for a shop where they offer fuel by hand pumps. The road is pretty good in some areas and has been improving. At some points, especially up near the Tak/Mae Hong Son border, there are a few bumps and holes which get worse during the rainy season. There can be mudslides during the rainy season as well.
Mae Sariang is not very big, so you can easily get around walking or renting a bicycle (50 baht/day) from one of the local guest houses. Motorbikes are available for around 200 baht a day as well.
For longer journeys out of the town, you may want to hire a car and driver. Ask any of the local guest houses and they may be able to arrange this for you
For those who are in Mae Sariang in the month of November: Try your level best to visit Mae U Kho, which is about 2-3 hours away. During the month of November, the hills and valleys of Mae U Kho, one of Thailand's most naturally scenic areas, turn to gold when the Dok Bua Tong (Giant Wild Mexican Sunflower, Tithonia Diversifolia) comes into full bloom.
Besides that, there are flower pastures, hill paddy fields and one of Thailand's highest waterfalls: Mae Surin waterfall along the scenic valley route to Mae U Kho. Public transport is scare, you may need to hire car in Mae Hong Son. Weather is quite cool, you may need to bring along some extra clothes (especially bikers).
Mae Sariang, beyond offering visitors areas of exceptional natural beauty and insights into traditional tribal ways of life, has some notable ancient holy sites; where tourists are welcome to join the locals in paying their respects for good fortune and prosperity. These sites are together called the Phra That Sii Chom (The 4 ”Chom’’ reliquaries).
Hilltribe farming rice
Karen & Lawa Hilltribe villages: One of the cultural highlights here are the authentic hilltribe villages scattered all around Mae Sariang. They will not be signposted so may be difficult to find. Thailand Hilltribe Holidays offers guided tours (trekking and by car) of the area. Specializing in responsible travel to the villages and cultural immersion into the local way of life 
Mae Sariang Museum: Near the turning to Ban Chom Chaeng at the intersection of Hwy 108 and Hwy 105, the museum is a single-storey raised wooden building in a design which blends traditional Thai Yai wood-carving with northern Thai architectural styles. The museum is a centre for the display and sale of One Tambol one Product (OTOP) products, and features presentations about the way of life of tribal peoples including Lua, Thai Yai, Karen, Lawa, and Hmong along with local history. It also has plenty of information for tourists about sites throughout Mae Hong Son.
There are far, far fewer tourists making it to Mae Sariang than Pai, etc., so you can be sure that you won't run into any other tourists, and there are no "ethnic shows". For the moment anyway, a much more genuine hill tribe experience. The local villages are much less visited so you may not be able to find a tour leaving during your time in town. Most operators require a minimum of 4 customers per trek. Also be prepared to pay a little more than you would in Chiang Mai or Pai. The cost is outweighed by a much better trekking experience.
Salawin Tours is a trekking agency in town. Run by the charming Salawin out of his "office". They can organise single- or multi-day treks, elephants, and rafting.
Some of the hotels and guesthouses have also started organising single day trips- North West Guest house, tel 0897009928/0866704286, has great tours for 1, 2, or 3 days. Kitti and Tukta are the local tour guides at North West. They know Mae Sariang well and will show you all the local sites. Very friendly people!
A day trip to Mae Sam Laeb is worthwhile. An hour away by Song Thaeu or motorbike, it is a border village overlooking Burma and the Salaween River and is home to Thai and Karen and Burmese alike. 600 baht gets you a one hour boat trip up the Salawin. 
There are two really nice temples on hilltops within 30 minutes by bike. Make sure to pay a visit to Wat Chom Thong.
Wat Chom Thong: Wat Chom Thong lies south east of Mae Sariang. Take Rural Road Mae Hong Song 4057 out of town and follow it crossing the Yuam river. From the bridge you can already see a giant golden buddha statue on a hill in front of you. Follow the road until you get to an intersection and then take the road through a thai-style gate leading uphill. After a good climb you will reach the temple. You can park your bike at the thai-style gate, if the climb is too steep for you. From up the hill you can enjoy a magnificent view and can take a walk around the temple, featuring some really nice flowers, the giant buddha and a lot of beautiful dragons.
Wat Phrathat Chom Mon: The Wat lies north west of Mae Sariang. Take the main road with all the big hostels, like riverside and north-west guesthouse out of town, going north. At the end of the road take a left turn. You should cross the Yuam river within minutes. From here you can already see a big golden chedi on the hill to your front-right. At the second intersection go right and follow the road, passing Ban Phamolo School. The entrance to the Wat is now straight in front of you. Take the steps up the hill and enjoy the sight onto Mae Sariang.
Muay Thai, 311/7 Moo 1 Bantakam (3 km north of the centre, very near Rt 108), ☎ +66 85 712 6422. Private Thai boxing lessons. The teacher, Meopar, aka Mr. Suphachai Sawakgul, is a former Thai boxer himself. He's an incredibly patient teacher and is very flexible in his teaching hours. Just give him a call and you can probably show up one hour later. Best hours are in the morning though (to avoid the heat). His English is basic but just enough to understand each other. The lessons are held in his garden on a Thai boxing mattress. Boxing gloves are provided, but bring your own (short) pants, towel, and a bottle of water. GPS-coordinates: N 18° 11.386 E 97° 56.109200 baht per hour. edit
To get to the hot springs outside of Mae Sariang, you'll need your own transport. Drive east on Rt 108 to Chiang Mai, turn left after about 18 km (GPS: N 18° 09.699 E 98° 01.987), drive another 13 km on an OK road until the signs (GPS: N1 8° 12.290 E 98° 00.620) indicate to leave the road to the left (it's right in a steep corner to the right if you were to continue on the road). Then drive another 2 km on a track (not too good) and when the road splits into two, turn right on the track that leads down. There you can park your vehicle and walk over the bamboo bridge to the hot springs. The people are friendly and speak barely any English. Bring your own towel. You can buy a few basic snacks and drinks. You'll get a remote feeling of the place. The setting and scenery is special.
Inthira The largest and most popular restaurant in Mae Sariang. Large variety, great portions and superb food. The owners are lovely. Ti, the main cook, makes a great red curry. The Tom-Yum Goong (prawn soup) is also particularly good. Very laid back atmosphere, fast service, and can accommodate large groups! Highly recommended! Website here.
Leelawadi is on a crossroad before Inthira. This place is reasonably priced and looked like a family run business. It has an open road view and stuffed with all the antiques related to flying. They have an excellent vegetable fried rice. Rest of the food was also decent enough. The lady who cooks is very sweet and accommodative to your diet. On few days there is a market on this street.
River House Hotel. They do a truly excellent steak for 180 baht, and the open-air restaurant there overlooks the river. Opens for dinner from 5:30pm. Note this is not the same River House Hotel - which is next to Redwood and Coriander restaurant and is more up-market.
Sawadee, overlooks the river is excellent and they stock BeerLao for only 80 baht a bottle (33 cl). The owner has excellent English and she cooks a good, if slightly small American breakfast. If you like, or want to try, Isaan-style Som Tam, Sawadee is the place to try it. Ask for it 'pet nid-noi' if you are afraid of it being too spicy.
Tukki's Riverside Restaurant Sits on the opposite bank of the Yuam River right across the bridge on the south end of town. Great pad Thai with the best river view in town and the price is very decent. Has a pool table, new bar, and expanded menu.
North West Guesthouse, 81 Moo 12 Bankad (from bus station; go out, turn left, take first right, and right at the end of the soi. It's 20 m up on your right.), . Lovely, wooden, if basic, guest house near the river, and run by an enthusiastic and welcoming Thai couple. Has fan and air-con rooms. Great value for money, simple, clean, rooms. Very friendly staff and great atmosphere, a pleasure to stay in. Also has great food, made by the lovely owner Tuk-ta. All of it is delicious, especially good if you want to try some Thai such as pad pong curry, satay chicken, and salawin fish cake. If there is something you would like they will make it. And she loves to teach you to cook Thai food!Rooms 250 single shared bathroom, 300 double, aircon for another 50, and up to 400 baht for double air-con ensuite with Internet PC in the room. edit
Mae Sariang Guesthouse, (On the corner across the street from North West Guesthouse). Very basic accommodation, with poorly constructed walls, which means it can be noisy at night. But appears to be the cheapest place in town. Soap and towel are provided, but no wifi.200-300. edit
Ban Pangsariang Guesthouse, across from Mae Sariang Guesthouse. Nice family owned guesthouse with 4 fan rooms, each with private bathroom and hot water. They offer Thai food, coffee, and ice cream. Wifi was good, beds were very hard.300 baht. edit
There's a really cheap laundry on Th Mae Sariang Road, opposite of Wat Jong Sung adjacent to the Mitaree Hotel. One Piece 3 Baht, Underwear 1 Baht and Socks free. There's no sign, but you can see a washing machine standing in the shop.