Thai Visa Requirements?

I have a return ticket to Bangkok, departing in November and returning in May. I intend to travel round se asia and re enter the country at least three times before I go home in May. Should I arrange all my visas before I go, or get an onward flight from bangkok, or not bother with either. Just need to know if I'll encounter any problems at the airport if I don't have evidence of what I'm doing beyond the 30 days??