Is there any way to encourage my hair to curl...?

I have naturally straight hair that's pretty thick. In order for it to curl and stay curled, I have to use a small curling iron or hot rollers and huuuuge amounts of hair spray. And after all that, it only stays curled for like, 2 hours. Then it starts to flatten out. So I was wondering, is there anything I can do to make my hair more susceptible to curling and to be able keep the curl? Maye shampoos, conditioners, serums, sprays, etc... Homemade stuff would be awesome. Anything? Thanks.