Focusing on color and light:
* The sunset was like a burning ember fading in the sky.
* The sky was ablaze with colors like a painter's palette.
* The sun dipped below the horizon, leaving the sky like a splash of watercolors.
* The sunset was as golden as melted honey.
Focusing on the feeling:
* The sunset felt like a soft sigh at the end of the day.
* The sky turned the color of a lover's farewell.
* The sunset was as peaceful as a lullaby.
Focusing on the visual:
* The sun sank like a giant orange ball into the ocean.
* The clouds were painted with hues like a stained glass window.
* The sunset was a dramatic curtain call for the day.
The best simile for you will depend on the specific sunset you are describing and the mood you want to evoke.