I have extremely hard to curl hair, help?

I have very fine 2 foot long hair. When I try using a curling iron at maximum temperature, the best I get is a slight curl at the end of my hair that last 10 seconds at best. I swear I'm using the iron right, by the way. I've tried a straightening iron (didn't work at all), sponge curlers (works for about 20 minutes), sleeping in a bun (same result as the curlers), practically everything I've heard works and my hair still will not hold a curl more than 20 minutes. I do use A LOT of hair spray plus hair mouse. HELP?!?!