Make It Fun!
When I was very little I had trouble saying R's and L's properly and so my parents took me to a speech therapist. I remember I used to LOVE going as she made the lessons so much fun.
I had a scrap book and she had me cut out things that started with R's and L's out of magazines and then stick them in the scrap book so the next time I came to visit I could then tell her each thing I had stuck in the book. It helped me practice how to say the letters until I didn't have any troubles anymore.