In CS4 open the image in Adobe Camera Raw. You can do this with JPEG files as well as RAW ones. You just set it up in the preferences
Now with ACR open select the Adjustment brush. In in the tool bar above the image ( or cheat and just press "K" ).
Now you'll see a nuber of options in the right hand pane. You can adjust various parameters including saturation. Select a reasonable value ( you don't have to be accurate at this stage. At the bottom of the pane you'll see a tick box for auto masking, tick it. It makes all the difference as it will automatically help with masking the area you want and don't want.
Select a suitable brush size and start painting the essfect you want. Don't worry if it's to much or too little at this stage, just ensure you cover all the area you want.
If you make a mistake, pressing the alt or option key and holding it turns the brush into erase so you can remove any overflow that may have occured..
Now the adjustments you have made are not set in stone. If you need more saturation increase the slider, and the area you have worked on will show this increase in saturation. To much , yes you've guessed it reduce the amount.
You can also have multiple adjustments by selecting new from the panel. You can also mix and match. So you can add saturation and brightness at the same time.
The clarity control adjust mid range contrast, you can locally increase apparent sharpens. Like all the controls it will go negative, so you can use it to soften portraits should they need it.
It's not until you press "Open Image" are the adjustments "Hard Wired" and Photoshop process the image with those adjustments.
If you need more information Look for tutorials in using the Local Adjustment Tools in Lightroom. It's the same options but the interface is slightly different, but nothing you can't follow
