how to get straight hair for a night out?

i have really thick, naturally curly hair that i want to straighten for a party tomorrow night, i find it needs to be straightened a night in advance so i have a night to sleep on it and take out some of the volume. my problem is i have work in a cafe tomorrow so need to tie it up in a bun. How do i keep it straight but tie it up???? is there any way i can not get a big kink in it from the bobble?