Like the Anon ladies said, your best bet is to use a round brush and a blow dryer. Also use some defrizzing serum or spray. A blow dryer causes just as much damage to your hair as a straightener - I don't know if that's your reasoning for not wanting to use a straightener or if you don't have one or whatever.
heat styling mousse, round brush. you have to put the brush under a section of hair and apply the heat to the top of your hair as you pull the brush down. it's irritating as hell and will take forever. but you can get really nice volume through this method as opposed to using a straightener. I recommend using a heat defense shampoo and conditioner if you plan on doing this often...or else your hair will become super brittle and just break off :(