Visa' s and travelling?

I'm going off to Thailand to dive and master poi (fire dancing) my problem is the whole visa situation, apparently i can arrive in Thailand, they'll give me a visa and its valid till the end of the month. there after i have to leave the country, renew my visa and come back. the only problem with that is i have heard they've changed the return time for visa to 14 days before it must be renewed again, and i'm planning to be there until December. i can apply for a 3month visa here, but there are big issues with my air ticket being a return flight scheduled until December, which wouldn't be allowed with a 3month visa because they have to know what i am going to be doing during my stay. so to clarify here. is the Thailand visa once renewed only valid for 14days and what do you think i can do. i'm leaving the 30th march so i must act quickly