This helps both with discouraging tongue thrust and encouraging lip closure.
This can be used on children who are very young - from the very first time they start eating solids! Hold the spoon sideways, so that the side of the spoon is touching each side of the mouth, let the child get the food off it that way. Immediently turn the spoon the other way (still sideways, but facing a different direction) and feed the child. Immediently Turn the spoon again and let them get the last bit of the food off. You do this 3x in a row, because then it helps keep that tongue in. If you just do the sideways spoon feeding once, then the child can put his tongue out to help swallow the food or lick the food off of his lips. After you do it 3x's the child will hardly stick his tongue out at all after that time. You can do it to yourself.
Try it on yourself - Feed yourself with the spoon like you normally would - your tongue protrudes forward this way. Feed yourself with the spoon sideways, your tongue is pushed back. It works so well.
Below are several pictures. They go in order of how the child should be fed.