When you're creating a composite image, it's a game of balancing and matching a ton of parameters to make the different elements convincingly look as if they all originated in the same frame. This helpful tutorial will show you some of the most important aspects you need to have mastered to create your best composite work.
Coming to you from Nathaniel Dodson of tutvid, this video will show you the ins and outs of blending and matching lighting and color between elements of a composite image. While there are all sorts of things you need to check and possibly adjust when working on such projects, I think that lighting and color are probably the most essential, as they're so global that most anyone can immediately spot when something isn't quite right. I particularly appreciated this method as it relies less on eyeballing and more on using Photoshop's built-in tools for precisely measuring levels of color and luminosity. I know it's difficult (at least for me) to eyeball such things, particularly when they're not off by much, and obviating the need to look back and forth between elements tends to get the job done more quickly and accurately. Give it a try!