"Here is your tea mistress," Rarity said, dressed in a frilly french maid uniform.
"Thank you Rarity darling," Rainbow Dash said taking a sip. This was a lot different than Rainbow had envisioned. After Rarity had lost her bet, Rainbow Dash got her own live in maid for a month!
Rarity for her part... didn't mind at all. Helping others and giving her best effort came naturally to her. Cleaning up, washing, organizing, scheduling... It was felt so naturally to her. Then came phase two. Exposing Rainbow to a little classical music here, showing her some pegasus vintage artwork there. Making her some refined meals. Giving her some nice relaxing mane washings... Providing readily available feedback for her stunt work, subtly becoming more graceful and elegant. And finally, the beautiful dresses.
One morning, Rainbow Dash had looked into the mirror, seeing her decorated eyes, her curled mane, her lady-like poise, and hearing her own prim and polite voice, only to realize Rarity had turned her into a proper lady without Rainbow even realizing it! And... Rainbow Dash didn't mind.
Rarity had lost the bet, but she had won the war.