There is a thread around here somewhere with a lot of chat by Robin about Smart Objects in Photoshop. Unfortunately I can't find it to contribute this little nugget. If I remember correctly, she pretty much covered how to use Smart Objects and some examples. I've an example of using Smart Objects to help solve a problem.
I'm currently working on a build and I'm being very mindful of the colours and other things being used. I'm really flexing my muscles on this one.
I've got this wall and this floor. They look very good together. The problem is that I'm having a hard time adding the 2nd wall. I can't seem to nail where the colours of the new wall should exist. I've got the basic wall, but I can't seem to make it blend well with what I already have. Too bright, not enough saturation, hue slightly off - ugh! This is compounded by the fact that PS and SL are *very* different environments. Does it look good in PS? Yes it does. What does it look like in SL? It looks like crap.
Smart Objects to the rescue.
Take a snapshot of the build. For me, the snapshot includes the floor, the first wall, and where I want to put the second wall. Get this into Photoshop.
Open the wall that I want to add in all of it's 2d glory. Get it into the snapshot of the build as a Smart Object. Transform it into place using Distort or Perspective or whatever. The placement doesn't have to be perfect.
I can now get crazy with the new wall in it's 2d glory and see it reflected in faux 3d right in Photoshop. Instead of going back and forth between PS and SL, I'm going back and forth between 2 documents in PS. Once happy, do that thing you do, export, upload, and smile.