Oh boy, that was a tough one for Roscoe too. Still is sometimes, it takes a lot of work, especially for an alpha dog like him. Keep practicing the sit stay, but stay right by his side. When you release him (say all done or something like that, whatever command you choose for your release command) give him a treat. When he seems to get that far, face him and take one step back. Always follow with a treat and tons of praise. He'll get it, eventually. Make sure he's got that step before moving to the next one, my problem is that I try to move to fast. I am now able to make Roscoe sit stay until I go into another room for a while, and he will just sit there knowing I'm holding a treat, but it has taken tons of practice.

Another thing our trainer recommended was starting him doing sit stays while you are watching tv. Make him sit and stay during one commercial. (Then follow with the treat and praise, always). Once he has that, do it for two commercials. Keep going, until you have the whole commercial break. She said a good goal is to make him sit stay for an entire half hour sitcom. I have actually been able to do that with Roscoe, even though now sometimes he'll forget and lay down lol.

Another tip, while he's sitting, continue to say 'good boy, sit.' That seems to help us. Good luck!

Edit to add: I don't know your situation, but if you have not done any obedience training with your dog I highly recommend it. It helped us out tons!