I've been studying tongue thrust quite a bit and wanted to recommend a really helpful website: http://www.pammarshalla.com/blog/2013/03/is-a-frontal-lisp-outgrown/
I've listened to Pam Marshalla's seminars and tried her advice/therapy guidance. I love her style.
Tongue thrust can be pretty tricky for parents to try and correct by themselves. I recommend finding a professional http://www.asha.org/findpro/ who has experience with swallowing therapy. I'm afraid a lot of time and money will be spent focusing on the wrong thing (articulation only) unless a professional is given the opportunity to spend some sessions focusing on the swallow.
I've been volunteering in my 4 yr old daughter's preschool class and can see many of them (almost all) have some sort of frontal lisp. But, from a listening/watching obervation only, and from a few feet away, I wouldn't be able to tell if their lisp was more developmental in nature or if it was actually cause for concern. My daughter is one of the "lispers" and even though I am terribly biased, I really do believe hers is the "cute" kind that will truly go away on its own. She is already starting to say some /s/ words correctly and I've barely even made mention of her lisp.
My other daughter, is definitely a tongue thruster. We've been seeing a professional who has been so helpful. She (my daughter) can do a correct swallow when probed and can also do correct sounds with a model/reminder. Much progress has been made - yea!
Anyway, watch for signs of your/a child being unable to move tongue without moving jaw ("try to touch your nose with your tongue" or " lift your tongue"); also, where is the tongue at rest? Jaw should be up and forward with tongue on the top of mouth (behind front teeth). One interesting observation is that if a child tends to have poor posture, low coordination in large limbs, he or she is likely not being very coordinated with speech either. Your tongue is like another limb.
Good Luck!
