Sometimes when I give Lenney commands during a training session, she is "spot on" (usually before meal time when she is most highly motivated )... then it's very easy to mark "yes" and reward. But other times, she is a little sloppy (eg. a slouchy sit, or moves a step or two on "stand", or is a little off to the side on "front", etc.) I'm never sure how to respond, and I'm sure my inconsistent responses are confusing her. If she technically executes the command, but is sloppy or her positioning isn't quite perfect, should I actually say "nope" and then repeat the command? I have tried this and often Lenney will then pull out her bag of tricks (a few different positions in rapid succession) to see if she can get it right. Other times, I will ignore a sloppy response and just immediately move on to a different command (in the expectation of getting better execution on it). I guess what I'm looking for is the best way for me to clearly and consistently communicate the "nope, that's not good enough" concept to her. I do realize that the problem stems from my own inexperience -- I'm sure I have inadvertently rewarded sloppy responses in the past. But how do I undo that?

I reward when it is right only, after a few non-rewards I move on to something I know will get a reward and go back and try the other to see if it has improved. If it has not improved I just put him up till the next day and start with the one we were having problems with and see how we do, if its now working I just put him back in the kennel and go think what I'm doing wrong and what I need to do to get the correct action. Most of the time it is just time to start over on the command one step at a time with lots of rewards.

Can't beat a good "Nope" as a negative.
Simple to undo past mistakes. An incorrect behavior ignored will extinguish itself.
When a dog pulls out it's "bag of tricks" that, to me, tells me the dog is not solid in it's behavior. That usually comes form the handler not getting across what it's asking for. Confusion for the dog so lets toss everything we know at the handler and hope it works.
Be consistent! Be fair!