i agree with all the above.
Just to add my own way of getting a dog to sit if it will only do it for treats. Show the treat, say Sit, dog sits, treat and praise. Do this several times in succession.
Then don't show a treat (so have it in a pocket) and say Sit, if he sits, excellent, praise and treat. If he doesn't, wait him out for about 30 seconds without saying a word......keep still.....many dogs do then decide to sit to see if it produces a treat.....if he does, treat and praise.
If not, say "ah! too bad!" and quickly show him the treat he has MISSED,

and walk away.
If you repeat this, he will almost certianly "get it" in the end. YOu can then gradually train him all over the place to "generalise" his training. Try if he will Sit if you turn your back and say Sit, and so on. I saw my trainer once get her Dobe to do all sorts of exercises in succession with her back turned, it was fun and showed the dog really knew the commands :)
I hope i have explained this properly, it's easier to actually be there to show you :D
Lindsay