How to get to Hoa Binh

There are regular departures throughout the day to Hoa Binh from Hanoi's My Dinh bus station. It costs upward of 33,000 dong (depending on quality of bus) and takes about two hours. The bus will drop you in town or near to the bus station (if you are heading on to Mai Chau). 
If you are heading on to Mai Chau, buses leave reasonably regularly from the bus station and cost around 40,000 VND. 

Heading back to Hanoi, buses depart regularly from the bus station or from across the road by the post office. Prices range from 30,000 upwards — be prepared to negotiate if the price isn't printed on the bus.
Buses directly to Mai Chau that drop off in town depart from the My Dinh bus station in Hanoi at 06:00, 07:30, 09:00, 14:00, 14:45 and cost 50,000 VND. 

Alternatively buses to Hoa Binh leave Hanoi every 15 minutes from 06:00 - 18:00 (from 33,000 VND) and from Hoa Binh bus station there are regular buses to Mai Chau for around 40,000 VND. The bus might drop off down the road from the bus station but if you make it clear that you're heading to Mai Chau they'll kick you off at the right place and point you in the correct direction. From Mai Chau town it's a 2-3km walk or 10,000 VND on motorbike taxi to the villages. 

Another option is to take any bus leaving for Son La or Dien Bien Phu which will pass by and drop off at the Tong Dau junction about 5 km from town (the road sign for Mai Chau town centre reads 'Pho Vang'). From there, a motorbike taxi will be about 30,000 VND, or more, and since you're essentially stranded, bargaining won't get you far. 
Be warned that some people have found it tricky to obtain a ticket from the desk at My Dinh and while it's possible to buy a ticket on the bus be prepared to overpay. 

Direct buses back to Hanoi depart from the bus station in town — although no timetable is available they seemed quite regular. Ask at your guesthouse though, as some will be willing to get the bus to pick you up. On our last visit we were informed that buses went back to Hanoi directly from Bac Village at 11:15, 13:00 and 15:00 but this could change regularly so don't rely on it. 

If you have no joy at the bus station, head to the Tong Dau Junction and wait for a minibus to come along. Passengers tend to wait in front of Com Pho Tuan Anh just past the Petrolimex station on the road to Moc Chau. There will be something coming along each way every half hour or so during daylight hours.
For both buses from the bus station and Tong Dau be prepared to pay 100,000 VND to get to either Hanoi or Son La — you might have some bargaining power if you pick the bus up at the bus station but that depends on how long you're willing to wait for the next bus.