While I talk a lot about strengthening the jaw when it comes to helping speech, working on the tongue & lips is also important. If you work on the jaw, the tongue & lips will improve. But, they still may need some extra nudging.
This is one exercise that we do regularly with O. It can be pretty hard to do and there are multiple levels of it.
Take a popsicle stick or a tongue depressor. Tongue depressors can be easier than popsicle sticks, as they are a bit wider. A lot of therapy supply stores sell tongue depressors.
We happen to use grape flavored ones, as that is the kind we got from one of the first ST's we had.
Place the tongue depressor in between the child's lips and have them hold it there. Make sure the child's lips are on it all the way. It takes a bit of concentrating sometimes to get O to have his lips fully on it & holding it.
There we go, this is a much better hold!
You can either just have the child hold the stick in between their lips. Or, you can step it up a notch and lightly pull on the stick to give some resistance. You can also tape a penny on each end of the stick to add some extra weight, which therefore makes a more interesting dynamic for the child to hold it in between his lips.
This will all help greatly with keeping the lips shut, help with tongue retraction & to also help decrease any drooling problems the child may have.