Some people find teaching Bark and then Quiet as a pair works. I taught Romeo to bark on command (the fastest thing he learnt) and taught Quiet too. It works sometimes, usually when he's not overly excited (like when he checks out a noise at the gates and alerts me to it - he's still fairly mellow in the beginning after a couple of barks and Quiet works if I catch him early!) and not in one of his barking fits (attention seeking mostly). But it could also be because I haven't gotten into the habit of clicking and treating and reinforcing the behaviour when he does shut up.
When he gets super aroused/excited and is in a barking fit, he can barely hear me (yelling Quiet at the top of my lungs), so I take out the can of coins (which is like a gift from the heavens) and give it a little shake. You can experiment with what sounds make your sheltie stop in his tracks. That's just all you need - a sound that can cut through their barking fit and jolt their little brain to go "HUH? Whazzat?" Could be a can of coins, or clanging 2 pot covers together (as seen on It's Me or the Dog), or an out of tune violin? As you make the sound, say Quiet and reinforce with treats when they stop barking.