If you intend to stay in Thailand as a tourist for longer than 60 days (or will be using Thailand as a base to visit other countries in the region) you may consider applying for a multi-entry Tourist Visa from the nearest Thai embassy/consulate in your home country. Depending on your request, this visa will grant you 2, 3 or 4 entries to Thailand (a 4 entry visa will be more expensive than a 2 entry visa).
It is important to note that this type of visa has a validity period of 6 months from the date of issue. You must enter Thailand on your final visit before expiry of the visa. For example, you apply for a 3-entry visa on March 1st. The visa is processed on March 5th and sent back to you a few days later. Along with the embassy stamp, the visa will be marked with something similar to this:
Visa Number 12345
Category TR (Tourist Visa)
Good for Three (3) journeys to Thailand
This visa must be utilized before September 1st
Date of Issue March 5th
This means that you must enter Thailand on your third and final visit before September 1st.
Multi-entry visas may be extended at the sole discretion of the Thai immigration office nearest to where you are staying in Thailand (current extension fee is 1900 Baht). At the end of each 60-day stay (or extension of) you will be required to depart Thailand. If you possess a further valid unused entry on your visa and the visa has not expired, you can re-enter Thailand for a further 60-day stay.
At the end of your final 60-day stay (or extension of) you must leave Thailand and cannot re-enter until you obtain a new visa.