Having manipulated photos for quite some time now I have picked up a few tips and decided to make a "tutorial" about the most important steps of creating realistic creations. Imagine having a picture of a person that you want to move to a different background. If you simply cut and paste, it will not look believable. Here's some things you need to think about:
The light on your model must match the light on your background. The most important part is the direction of the light. If the sun behind your model is on the left side of her, it's natural for the rays to hit the left side first. Since the sun in this case is BEHIND the model you need to imagine how much light would reach the front of your model. If the sun was directly behind her, very little would reach the front and face. Since it's on the left, some light will reach the edges of your model and spread a little around them. You must be aware of your light source at all times.
Study the shadows on your background scene. Study the length, strength, direction, color. These shadows have to match the shadows your model will cast. You will have to create that shadow yourself so you must be aware of the surroundings. You also have to remember that when working with either light or shadows you have to pay attention to the way they interact with each other.
3. Smooth edges.
You have placed your model on the scene, now you have to get rid of those terrible harsh edges. We want the model to look like a human being in her natural enviroment, not a paper doll paste into a different scene. Use the smudge tool on a low setting and carefully go over only the edges of your model until she blends in nicely with the background. Do not go overboard with this.
4. Tone matching.
A common mistake is to not pay attention to the temperature and tone of the model as well as the background. If the background have a colder temperature with a blue tint, your model can not have a warm color tone. You have to match them. The quickest way for this is to use photoshop's color balance. You can also use "photo filter". Another thing you must match is the saturation and strength of color.
5. Realistic hair
Creating realistic hair on a photomanipulation can be extremely challenging. Masking out strand by strand will more often than not work and also take tons of time. Try something different, erase the parts of the hair that isn't needed and then use the smudge tool to "drag" out strands. A simple brush for this is photoshop's "natural brushes", the first choice. A little useful brush that can help you on your way. Make your brush thin and begin to drag out strands with a smooth hand while imagining the natural flow of the hair. This brush is not the best you can find for hair, I advice you to go to www.deviantart.com and search for a smudge hair brush. Don't forget to credit the creator!
6. Ground your model.
By adding very little shadow just under her feet you will avoid her feet looking like they're floating two inches above the ground. Blur the thin black line you just painted under her feet (on a different layer) using a low pixel setting.
7. Don't be scared of looking closer.
Inspect. Zoom. Go over little areas, especielly inbetween fingers and corners. Look for stray pixels, hard edges, forgotten pieces of the previous background. Do all this BEFORE dropping any layers. Make sure it looks clean and realistic, and take your time.
8. Match textures.
Look at the "texture" of your background and then your model. Is one noisier or softer than the other? Do they look "unmatched"? Now you have to experiment with different fixes such as using blur on different blend modes, adding or removing noise, sharpen, smoothing the skin using the smudge tool or gaussian blur.
9. Outside elements.
This part can be a bit tricky depending on how you want your final picture to look. You want your model to interact with all of her surroundings and elements. For exemple if it's raining, your model would naturally look wet. If it's windy, her hair will be blowing in the air as well as the fabric of her clothes. Is she standing in grass, then the grass will naturally surround her feet. If she's holding something that you added, it will cast a shadow.
10. Subtle added texture.
Adding some texture on your final picture can help putting your picture "together" as well as cover small imperfections. It can also create a very special mood. Experiment with blend modes and strength.
Some of these steps take training to really master but it's hard to create a realistic manipulation without paying attention to all of them. As always you have to experiment to discover new things and improve, be patient and have fun creating!