First off bring up your image in Photoshop or Elements and do all the retouching you feel it needs. I was going for a softer look with this image so when I retouched the rose I made sure the exposure wasn't too dark. Then I took out the blemishes on the petals and slightly painted over the yellow in the background. Next I added a new layer above the original layer and filled it with 50% gray. At the top of the layer box I changed this layer to Overlay. Once you do this you will be able to see your original layer underneath. Now grab your paint brush tool and choose a soft round brush with the opacity set to around 10 maybe even lower. Now make sure your colors are set to the default of black and white. On this new layer paint with the black to make areas darker and then go back and paint with the white to make areas lighter. You'll get the hang of it pretty fast and what is really nice about this method is.....if you don't like what you are doing then simply fill the layer back in with 50% grey and start again.
Now that you have that down......try painting with another color. Have fun!