Oooookay. I am constantly, constantly saying "nice picture, but your conversion is flat" to people. Sometimes the flatness is worse than others. I know there are a variety of ways to do it, so I am going to share my favorite and tell you how to NEVER do it. :P
Hope this helps some people out.
Okay, first, we have our beautiful couple. They just got married, look how happy and cute they are!
Now, I just desaturated the image. That works right? No! Don't do that! Why? It makes your image muddy and yucky. Look at their lips, they are pale, particularly if you compare it to the other conversions. Now, this photo is not the worst desaturated photo ever, because it has a lot of depth to it already, but, you lose a lot of shadows and highlights when you desaturate. Never do it. Just resist the urge. :P
We are sad. Please do not desaturate us.
Okay.. What about the gradient map? Do you know how to find it? The best method is to go to layers, click the little yin yang symbol (as I like to call it) and go to "gradient map". I always prefer to work in layers, it makes life easier when you want to make changes. So here is an image with the gradient map applies and then just flattened. Be sure to compare it to the desaturated image of doom and particularly look at their lips, cheeks and other areas that should have definite shadows and highlights.
We're looking a lot better now...
Now.. if you want that baby to REALLY really look pretty... Let's add an curves layer! It's really simple, go to the yin yang again, and go to "curves". You are going to make it look roughtly like an s, with the bottom left curving toward the floor, the upper right curving toward the ceiling and a little dot in the middle. Different images call for different s-curves, so adjust to your liking, just watch those highlights.
Compare this image with the two images above. You can see a huge difference!
Wow, look how amazing we look! Now that's a conversion with some depth, baby.
Okay dokey. Hope this is helpful to explain what I (and other people) mean by the dreaded.. FLAT CONVERSION.