Aim to get a stronger behaviour and connection in the dog's head before
asking for the behaviour. I recently taught Jasper 'shy boy' (covers his face with his paw) by sticking post-it notes on his nose and clicking and rewarding when he tried to rub them off. When he got the idea that putting his paw over his face got him a reward, I introduced the command but for a long time just said the command as he did the action rather than ask for it. Then, within a session, once he knew that the 'lesson for today' was covering his face with his paw, I'd start asking for the behaviour and only rewarded when I
had asked. Once he'd got this firm link in his head he was able to differentiate it from other commands.
But I get lazy, and having got that chuffed feeling of teaching a new trick, slackened off and as a result he'll sometimes often offer a different behaviour. Invariably he'll forget what he's meant to do just when I say to someone 'Look what my dog can do!' which serves me right for showing off
The only other thing worth thinking about is how calm and focused your dog is at the time. If I ask Jasper for a random command before giving him his tea he'll get in such a tizzy he'll offer sit, bow, paw, spin and pretty much anything else he knows because all he can think about is food
I bought a clicker today and thought that might help but not so far. 
