Many naive leaders believe they have two ways to enact change in their followers. The first way is "the carrot" aka praising good behavior. The second way is "the stick" aka criticizing bad behavior. In my experience, the stick is extremely ineffective. I'd say the only time the stick has any positive impact is when the follower doesn't realize she's doing something wrong, or how serious that wrong thing is.
The goal of this post is how to use a carrot when you want a stick. First lets recall how to use a carrot effectively in general: Keep it timely, short, specific and often (the one minute manager is my hero). Any time you see a new good behavior praise it. When you praise, be as specific as possible so the follower knows both the behavior you are praising and the valuable impact of that behavior. It's easy to praise when things go well, but how do we praise when things are going poorly?
When things are going poorly, there's only one thing you can do - put your followers in a position where they perform good behaviors. Scope their work so they have no choice but to perform good behaviors. From that good behavior, apply praise. Even if that behavior is accidental, still give praise. Slowly, sometimes painfully slowly, you'll get more opportunities to give praise, and you'll get more good behaviors. Keep giving praise, eventually the fruit of your labor will pay off.
Praising weak performers can be a time consuming delicate process, but it's like lighting a fire: at first you have to work really hard to spark the tinder, then pay close attention while the kindling catches to make sure you don't lose your delicate flame, finally you'll have a fire raging, and you can lean back and relax knowing not only have you lit a fire, but that fire can be used to light other fires.