One of my favorite things to do is going over some older pictures and playing with them in post-processing - trying adding sepia, exclusion blue, and gradient layers on the picture to get a moody or somber feel. As with any of my experiments, sometimes it works, sometimes it doesn't.
Here's what I do:
1) Do all the initial work in camera raw (white balance, exposure, contrast, and whatever else I think it needs)
2) Always working in different layers, I adjust the levels and color curves.
3) I usually start with gradient map layers and add at least two. Then I tweak the style (overlay and soft light are two favorites, but I always try multiples to see what works for me)
4) Depending on what mood I'm looking for I'll add an exclusion blue or sepia layers on top for a soft, older feel.
Here are some examples of what I mean.
For these two pictures of downtown Boston, I added two gradient layers - one BW and one copper - then adjusted their opacity and style. And then I added an exclusion blue layer on top. The only difference between the top and the bottom is that for the top I used BW gradient layer in hard light, and for the bottom I used screen. And just for comparison, here's one without the exclusion blue layer and the original respectively.
And more example, because... like I said... photoshop owns me.
Two gradient maps and exclusion blue layer (she looked a little orange so...)
Different shade of exclusion blue
With an added vignette
"Ma'am, step away from the photoshop, now, before anyone else gets hurt."