Compositing is a great editing method to learn and now, with the advancement of useful technology in editing tools like Photoshop, is easier than ever.
Skies the limit with compositing when it comes to creating amazing and unique images but if not done carefully can also create some car crashes!
Whether you want the image to look as realistic as possible or prefer a more graphic look I tend to stick to certain rules when blending my images:
Perspective - I always feel it's important to follow the rules of perspective. Objects moved from one background to another should be the correct size and angle when added to their new background environment, so it's best to use images that were photographed from a similar angle. If your background image was taken from above, adding an object photographed from below will look odd!
Light/Shadow - Pay attention to the light in the images you are blending and also the contrast and exposure. Mainly you can address this in editing but if the pictures are too contrasting in light and dark it can be a struggle to maintain image quality. So again I find it best to pick images that have a similar light quality. In the images I have used below, although the exposure may be different there is not much light and shadow contrast in either image, so I was able to blend them without too much adjustment. Also try to add a shadow to help ground your object, if needed.
Some people may believe compositing to be a new thing, only achievable with modern computer programmes, but one of my college projects completely revolved around images I had blended in the darkroom by laying one negative on another and varying the length of processing/exposure time.
Here is an example of two images I have blended to create a composite image using photoshop.