Once you have your image elements selected and extracted from their source files, it’s time to start blending them all together. This is often the stage when photographers new to high-level compositing make mistakes. For a composite image to truly work, each element must look as though it fits organically within the new setting, which can be a challenge even when you have attempted to achieve uniform light and colour at the shooting stage.
One of the major improvement points is edge halos and colour spill. Light from the background can reflect onto the edge of your subject, outlining it with distractingly contrasting colour and tone. This makes it tricky to seamlessly blend the object into the surrounding detail. Halos can be reduced using the Decontaminate Colors option in the Refine Edge dialogue of Photoshop, while the Matting option can perform a similar function (Layer > Matting > Defringe).