Boudoir hair, don't care.
Big hair for everyday life may have died in the 80's, except when it comes for boudoir sessions (maybe keep the crimping iron and bright blue eye shadow at home, though). Whether you have hair/makeup done for your session, or (especially) if you want to do your hair yourself, notice how 'big' hair is in boudoir photos! If you have thin hair, you can fake it with teasing, hair spray, and a curling iron! Do your homework and watch YouTube tutorials. Don't do this to your hair everyday, obviously, but just this once - you won't regret it (peer pressure, everyone is doing it, etc ;)). And don't worry if you're afraid of it falling flat - I will take care of you and help you fix it, toss it around, or run your hands through it to make it look fab again ;).