Yeah, I trained Jo pretty much the same way. She learned it in like 2 mins, it was kinda scary, lol. I just said "Josie, sit" then I would push her down to a sit, and leave her for a second, if she stays, I'd praise her with love and a treat.

A saw on a show once, for anyone wanting to teach their dog to lie down, is that you get him/her to sit, then take a treat (saying lie down) and show him/her it and slowly bring it under its chin and down its neck kinda, the dog will try to get the treat, the only way doing so is to lie down. Once he/she lies down, give the treat and praise. Work wonders for me