
Just like most people, I went through the phase of thinking selective color was really cool. Now it all seems so cliche, but when clients request it, I'm not too proud (or well-off) to refuse to do it. Gotta pay the bills. One thing I have found that helps the effect is to be more gradual about it. Most people's examples of selective color will have a completely destaurated image punctuated by a gash of color that seems like it was over-saturated due to the contrast with the monochrome. Dial it down a bit. Make the color very subtle so that at first, your viewer isn't even positive that the element is indeed in color.
That's just my two cents. Wish I had an image to show you what I'm talking about, but I'm at work, wasting my employer's money.
+1 on this thought. I like selective color when you have to do a double-take to see it. More of the pastel look, almost like the hand coloring of B&W photos in the "old days" when I was a kid...