The advice I've always heard (and my 5 yo has troubles with "R"s) is to model what you want them to do. So be sure to say those words clearly, make good R's and S's in your speech so they hear them but the advise was also not to "Correct" them, just model. I'm not sure of the rationale behind that advice I assume it has something to do with not making them self conscious about it. I will say that my son's abilities are improving, so I don't know whether it's modeling or just natural progression, but it's improving. He starts Kindergarten this fall so I plan to discuss it with his teacher and see if they want to ride it out or get him some professional help.