Excellent advice, Al, to which I must add:
8. Shadows can make or break a chop. Bright light = crisp-edged shadows; diffuse light = fuzzy-edged shadows. Use Gaussian blurs and transparencies to achieve realistic shadows. Use shadows in the source image as your guide. Keep those shadows in their own layers.
9. If you mirror an image to get the correct light source direction, good for you. Just know that if any text or noteworthy symbol gets reversed in the process you will need to repair that.
10. Beware clone tool patterns. To cover an object backed by sky or foliage, for example, it is often better to cut and paste a patch (with a generously feathered edge) of the background into a new layer over the object . The layer can then be tweaked with saturation and lightness/darkness adjustments. Conservative use of the clone tool can then be applied after the layer is merged downward.
11. When cutting out elements with the lasso, do small sections at a time. There is nothing quite as frustrating as losing a complex cut because your middle finger farted on the right mouse button.
Remember, you need to use the source photo in your chop. Here are some other things to remember:
1. A story is important. Just adding random things to a picture only tells us that you can cut and paste. A final image that tells a story, that makes sense will always do better.
2. Humor helps. Make 'em laugh, that counts too.
3. Before you start, turn the source photo upside down. Sometimes good ideas come from a different point of view.
4. Watch the light direction. If the source photo is lit from the left and your addition is lit form the right it won't look right.
5. Feather your cut outs. A pixel of feathering eliminates the jaggies.
6. If it doesn't look good when you are finished don't post it. If you label your entry with "This isn't very good but . . . " We'll believe you.
7. Have fun. It's why we are all here.