i have worked with kids with downs and using sign languge with the word helps make associations for them also make it fun not a task and have rewards for doing well and improving.
My daughter has Down Syndrome. I recommend a licensed therapist, first and foremost. But, some things the therapist had me do: touch ice to her tongue in play times of 15 minutes or so. This caused her to pull her tongue in (unless she was hot and thirsty). Also, talk to her--looking directly at her. And talk to her all the time--in non-baby talk.
Additionally, we used simple signs from sign language (eat, drink, toilet, please, thank you, sweet, sorry,) until she was verbal (around age 6)
hope that helps.