Of course there are more and more ATMs popping up every corner of tourist destinations especially in Yangon and Mandalay.(To see the list of available ATMs at certain cities : please click here. ) However, there won't be much accessible ATMs in other towns as Bagan and Inle Lake. Worse if you are travelling off the normal touristy route. As ATMs are introduced awhile ago, it is not that reliable to perform a transaction through these ATMs as there might be system errors and customer services for ATMs are not that well trained nor equipped. Credit cards are accepted in several hostels and hotels but there are others that only accept cash. Therefore, it leaves tourists no choice to bring enough cash with them to travel to Myanmar.
Credit cards are accepted in high end hotels and restaurants in Yangon and Mandalay. However, if you are considering to withdraw money from your credit card through hotel, it would not be so convenient and cheap but it is possible at hotels like Shagri-La and Chatrium. Travellers cheques are not popular at all in Myanmar.
Alternatively, you could book and pay online for at least your accommodations and flights or get a travel agency to book it for you and wire the money to them. In that way, you will not have to bring so much cash with you while travelling.