One of the very best ways to get kids saying new sound or new sound combinations is to have them watch you as you say those sounds. Pulling out a mirror so they can see their own oral motor movements is also an excellent idea, especially for these kinds of sounds: b, m, p, th, f, v, l, and s. This kind of activity works best when your child already knows how to make their new sound and needs a reminder to use it in their target words.
Here is an example: child can say the "s" sound but he often replaces it with a "th" if he is not paying attention.
Get in the bathroom in front of a mirror big enough to see you both.
Mom: Sssam, see how my tongue stayed behind my teeth! Now watch it will slip out this time: "tham", oops, how do I fix it?
Child: Pull it back behind your teeth!
Mom: Okay, Sam. How was that? Awesome! Your turn:
Mom: I noticed that your tongue was behind your teeth!
If your child is having a hard time getting the right sound out at all, this activity might not be as successful. That is when a knowledgeable expert (aka Speech-Language Pathologist) is super useful!