Photoshop :: Merging 2 Pictures Together

I do a lot of image editing, example cutting out pictures of a persons's head and place it on a picture w/c contain a body such as a king, princess, soldeirs etc. I often cut out pictures from digital shots and put them on a normal .jpg file to merge them. The issue is that the color profiles/brightness/contrass of the 2 pictures are not the same so it's very clear that they are edited.

I tried them all, under "Image-Adjust" then changing the image's hue/saturation, color balace, brightness/contrast however most of my works are still very noticeable that it's edited. only a handful of few shots comes out really blended that well.