Yet another creative way that SLPs think, get (my) kids to work hard, and allow us to all see progress: lip blocks and silly straws!
When combined together, they provide tactile cues to encourage lip closure, lip rounding, and tongue retraction.
Your SLP will tell you if/when your child is at an appropriate age for these products, and they will instruct you as to the position and length of the straw.
Just so that you know, if your child is able to bite on the straw that is attached to the lip block, then the straw is most likely too long.
The silly straws can be purchased at any dollar store, and I personally bought the lip blocks at Talk Tools.
So far, I am getting a little resistance from the boy to the lip blocks, but blowing bubbles with the silly straws prior to using the blocks have helped!