Partial Desaturation (Tanya Leibman)

1) find a picture with something worthy of being isolated from the rest of the picture, such as a flower or an eye

2) duplicate layer

3) go into quickmask mode ( Select > Edit in Quick Mask Mode)

4) use the paintbrush tool to cover what you want to stay colored. you can use the eraser tool to fine tune. you can experiment with the size, opacity and hardness of the brush (i.e., a flower might be harder to trace, so maybe use a softer brush, vs. something more defined use a harder brush)

4) Select > Inverse

5) Image > Adjustment > Desaturate. You can also play around with other option

like lightening the background instead of desaturation, or playing around with hue etc.