Compliment (reward) her for positive things.
Don't compliment for negative things.
Think of it as training a puppy...
If she started working out that week, and she's lost a few pounds, then compliment her some way on she's looking good: "those jeans look really good on you". Don't bring up the fact that she's been working out - let her connect the dots - that way she'll decide on her own that she wants to go to the gym more. If she cooks a good meal - give her a compliment. If she buys you something - give her a compliment. If she nags you for no apparent reason - withdraw attention and any sort of compliments or rewarding behavior.
Disclaimer: There are beta ways to compliment and alpha ways to compliment. A simple "you look really good today babe" or "that was a great meal" can be all you need sometimes. Give her a little appreciation and recognition for her good behavior (but don't over do it) , and she will do more of it - and aim to please you.