By plane
Bangkok Airways (http://www.bangkokair.com) now has a daily flight from Bangkok Suvarnabhumi airport, which then continues on to Lampang. Fares from/to Bangkok start from THB 1700 1 hour 20 minutes, from/to Lampang - from THB 1100 around 40 minutes.
By train
Take the express train 7 hours from Bangkok or Chiang Mai to Phitsanulok and go by bus from there 1 hour.
By bus
Sukothai only has a small bus station, but the city can be easily reached from the North, South, East and West.
From Mae Sot: Minibuses run regularly from the station behind the market. The journey takes about 3 hours, passing through Tak on the way. 130 baht.
From Phitsanulok: Frequent buses operate from the main bus terminal and take about 1 hour for the 58 km trip.
From Bangkok: There are direct buses from Bangkok Mochit Terminal and takes 7 hours, including some stops at Bus Terminal of major cities on the way.Understand that at the Terminal Mo Chit, there are several bus companies travelling to a particular destination. In this instance, Wintour travel, as at November 2010, charges B326 per person for a first class aircon bus. It takes about 6 hours.
From Chiang Mai: Buses from the main bus terminal take about 4 hours. The cheaper local buses make many stops and take about 5 hours.