Look for your local fly fishing club; some of them have casting clinics. I went to a casting clinic held twice annually by a club I belonged to when I lived in Vancouver, Washington, and in about 10 minutes, my roll cast went from a pile of line 15 feet in front of me to a solid 40-foot cast, thank to the help of a more experienced club member.
In addition to SilverCreek's advice about the hammer, I was told that my arm should be moving faster at the end of the cast than the beginning. So slow, then fast.