Sing the syllables "Lee lie low la" 4 times ending on "lay" on the notes c d e f g f e d c twice through. This increases flexibility, improves articulation, & helps get the tongue out of the throat, where it tends to block the sound. Remember, the higher the tongue position in the mouth, the better. (I'm talking about the back of the tongue - the front of the tongue should tend to rest behind the lower teeth, though in practice it moves all over the place depending on the syllable being pronounced.