What should I wear on this very long flight?

I'm leaving on Tuesday from Sarasota, FL to Phuket, Thailand. The total time including layovers is 31 hours. I'm obviously a guy and I don't know what to wear, this is my first long haul international flight. I was thinking PJs but my girlfriend said no. Jeans and a shirt I think would get uncomfortable. What do you suggest? And are flip flops a good choice for a flight this long? Thanks.