Effective Nov 13th 2015 the Thai government will introduce a new 6 month multiple entry tourist visa available to all nationalities.

This new visa type will replace the old Double and Triple entry tourist visas.
A single entry visa can still be applied for if required.
The visa will be valid for 6 months from date of issue good for multiple entries for stay up to 60 days on each entry.

Extension of stay: Those who wish to stay longer or may wish to change their type of visa must file an application for permission at the
Office of Immigration Bureau located on Soi Suan Plu, off South Sathorn Road, Bangkok 10120, Tel (662) 287-3101 to 10 or at http://www.immigration.go.th
The extension of stay as well as the change of certain type of visa is solely at the discretion of the Immigration officer.