1. Multipass rendering. Now that's awesome for touching up in PS.
2. Try throwing the scene into Maxwell Render. You might be pleasantly surprised, and might learn a few things on the way.
3. Dust/rust shaders, Final Gather (assume you're using Mental Ray) or GI? The soft shadows desperately need to be there.
4. Extra grain and color processing in PS: Optional, but it's always there if you want a mood (for example green punched sepia)

lol Grain is film grain. When you use Maxwell Render the first few hours have a grainy render, then slowly the image clears up. And if your scene low-poly and low-lights, you can use Radiosity instead. Doesn't need any time to calculate when rendering.