I flew to Bagan from Rangoon. There are flights to Bagan - often via Mandalay from most airports in Myanmar. From Mandalay you can also sail on Irrawaddy to / from Bagan. It takes a whole day. There are also buses to / from Bagan from all cities in Myanmar e.g. via Mandalay. My favorite though is the train from Bagan to Mandalay. A 1st class ticket for the 7 hour and 180 km long ride on the world's most crooked rails costs 1.5 USD and the train runs through stunningly beautiful nature and many small villages where you can look in through the windows and follow everyday life.