I have struggled with hard-to-curl hair for years. It wasn't until a recent trip to the hair salon that I discovered what the problem was. My stylist told me that my hair was almost too healthy and in order to get it to curl, I would need to "damage it a little." Although I am by no means a hair expert, this has what has worked for me to give my hair a little extra bounce and curl.
Wash hair with volumizing shampoo. Skip the conditioner.
While hair is still dripping wet, comb through hair and apply a large amount of strong-hold mousse to hair. Comb through thoroughly to ensure even distribution throughout hair.
Blow-dry until hair is still reasonably damp.
Sparingly spray medium-hold hairspray throughout damp hair, taking care to reach the hair underneath and not just on the surface.
Blow-dry again while combing through hair. Combing through the hair is necessary to keep hair from drying stiff or sticky.
Curl as normal with curling iron on high setting, taking time to hold the hair a bit longer on the wand to ensure a stronger curl.