When you composite, you are almost always trying to match your foreground to your background plate. Obviously, a big part of this is matching the color and contrast between the two separate pieces, but there are other things you can do to help improve the blending between the two layers.
One thing that I find very helpful is doing edge blending. What I do is take my background and wrap it over the edges of my foreground. When it is feathered out and made partially transparent, the composite is much more natural and realistic then with color correction alone.
To do this, duplicate both your foreground and background and move them to the top, keeping the background duplicate directly below the foreground duplicate. Apply the 'Set Channels' effect to the background duplicate and set the 'Source Layer 4' drop-down to your original foreground layer. This will create a cut out of the background in the shape of the foreground. Then, on the foreground duplicate, apply the 'Fast Blur' effect. Set the 'Track Matte' option for the background duplicate to 'Alpha Inverted Matte'. That's it. By adjusting the 'Fast Blur' amount, you can control the amount of feathering. You can apply effects to the background duplicate to add any extra color correction to the edge wrap.
This setup gives you fairly basic control of the edge correction. There are other ways to get more control over your mattes. For instance, I have used the 'Edge Color Correction' options in the Keylight effect.
But no matter how you create it, blending the edges of your composites will help give you much more realistic results.