We are going through this same thing right now. DS is 2yrs, 8 months and has been seeing someone for about 3 months now. We started out with Early Intervention having him evaluated. They checked for everything, fine motor skills, language, speech, etc... Like Holly said, it was basically like they were playing with him. Thankfully he was at or above average on everything except speech. So once they figured out he needed help with that(like I hadnt been saying that for months???) they assigned us a therapist. Once it fit her schedule she started coming once a week. She spends an hour with DS and again, like Holly said, they "play". She comes with her bag of toys, and DS loves it. He waits at the door for her. She has said she uses the same toys a lot to reinforce certain words(up, down, open...stuff like that). Weve even done "face games" where we blow air through a straw to move cotton balls, or she holds a flavored tongue depresser near the side of his face so he will stick his tongue out to that side. Also, she has explained to us that each sound has 13 aspects to making it(the air, the voice, where the tongue is, how the lips are shaped, etc...), so if even 1 part of that is off, the sound wont be right. There seems to be something going on making that happen to DS.
I have definitely seen a great improvement in DS, especially within the last month. He still isnt talking great, and probably not at all where he should be, but he is making progress. It took a couple months before we could really notice a big change.
Sorry this is so long, but Im trying to include all the info I can think of. If you have any specific questions just ask.