Yes, if you already have somewhat curly or even wavy hair, with the RIGHT products and styling techniques you can achieve curlier hair!
Check out Naturallycurly.com for tips. It's really a great start!
As for some products: I like and use: AG Recoil, Boots Curling Cream, Biosilk Rock Hard Gelee (just to name a few). There are TONS more products out there. Look for products that are geared towards curly hair and that say "Curl Enhancer".
Hope that helps!