Tracy's advice is dead on - is the easiest way I've found to train for stay. It's how I have all my classes get started, and have never seen it fail. When you pivot in front of them, you get their attention and are right on top of them to stop any movement. Then immediately back and "good dog!". Don't start moving away - even 1 foot - until you can pivot in front of them with no reaction from the dog. Then gradually start moving away (keeping the leash in your hand). Biggest problem most people have is going too far too fast.
Once you can get a distance away for a good period of time, here's a good "proofing" method. You need a small mirror and two rooms separated by a doorway. Position the mirror when it can be seen from both rooms. Put the dog on a down stay, walk into the other room and watch the dog in the mirror. As soon as the dog starts to get up, yell NO! and storm back into the room, putting the dog back in the exact same spot, back on down stay. Leave the room again, watching the mirror. Most dogs wonder how in the blazes you knew they got up, and start to think you just always know what they're doing. My current dobe, however, learned to watch ME in the mirror. They're a bit TOO smart sometimes. Is a safe, easy method of teaching your dog it has to behave even if you're not in sight. Good luck! Carol