Recently, I've been working with frequency separation, and as with everything, the more I use it, the better I get at it. There are mucho articles and videos on the best way to use frequency separation. It's very useful and I've found it reduces the time it takes me to edit, as well as the number of layers I use to edit. This post isn't about how to do it, as I've said it's been covered ad nauseam. I will link you to the tutorial that finally convinced me to give it a try, because prior to this guy, it seemed like way too much work to me. You can find a tutorial as well as an easy action you can download to save yourself some time on FX-Ray,'s website.
In the action it's separated into low frequency which I use to edit color differences and high frequency which I use to edit detail, like skin, blemishes, wrinkles and such. The layer we are working with is the low frequency as in the pic below.
Once you're on the low frequency layer, go to your paint brush layer and select the mixer brush tool.
Below is a pic of the settings I use for my brush.
To use the brush, select the color area to sample. The shadowed area is what you want to get rid of, so select the area above it to sample. As with everything, be careful of your sampling areas because you want to make sure the area you paint matches the area around it. In the below example, I haven't done anything to the picture save brushed away the shadow on her lower stomach. Getting rid of that dark area 'flattens' out the small bulge that was there and now her stomach appears flat. The same thing can be accomplished using the healing brush, but when you're working with textured garments too much healing removes that texture. That's the biggest benefit I've found with using frequency separation, not having to sacrifice texture.
Another example. Nothing has been done to the image save removing the shadows on the stomach area. As you can see, the bulge is noticeably smaller in the pic on the left.
A super fast fix for a problem that plagues the lot of us. As with everything, personal preference changes with every person and this doesn't work on everything. I have found it useful, and it works for me. Hopefully you get some use out of it. Feel free to leave me questions in the comments.