How can i get nice waves that aren't poofy overnight if I have thick hair?

Well I have really thick hair that goes a little past my shoulders and at night after I take a shower I put my hair in a lose bun and just sleep in it. When I get up in the morning I take it down and wala! nice unpoofy waves...but usually as the day goes on, it gets reallly poofy and frizzy and what not.. so I would suggest putting a good amount of hair spray on it before you leave :)