You know that saying, "You can't get blood from a stone?" If your hair doesn't have the ability to produce root curl, you won't get it no matter what you do. That being said, all of the recommendations above may help you get root curl if your hair has the potential to curl at the root.
I've gone to the same stylist for 30+ years and have gotten very good haircuts for all of that time. I didn't use products until I started to lurk on nc.com (my stylist sprayed Mr. Right on my hair in Feb and I had a little curl instead of my winter waves.) It got me started on my journey. I learned about my hair, what product and technique it liked--and now my hair is curly (with root curl) all of the time.
3a (Corkicelli), highlighted, fine, low porosity
modified CG, since April '07
CG since 3/11/08
HGs: Anything Sevi; Curly Kinks Satin Roots, Curlycue ReNew and Coil Jam; homemade FSG and okra gel; soap bars; UFD Curly Magic; Botanical Spirits Jellies, CJ Repair Me