In order to make the image as realistic as possible, a few less obvious changes bring the whole image together and make it more believable.
Lighting
The most obvious giveaway that an image has been faked is inconsistent lighting. Use the Levels Adjustment to alter the contrast so it matches the overall tone of the image. I opted to leave it darker because it gave a feel of being in the shadows of the palm trees. You may also need to flip the image horizontally (Edit>Transform>Flip Horizontal) to better match the light direction of the background image.
Color
As with the lighting, the eye is good at picking up color discrepancies (but not as good as it is at the lighting). Use the Color Balance sliders or Hue and Saturation to get the desired overall hue of the images to match.
Depth
You can decrease the focal length to give a more telephoto appearance to the image by adding a slight blur to the background image. You can even go a step further by feathering a selection to give less blur to the parts of the background image that are closer to the subject.
Grain
Lastly, flatten the image and add just a titch of grain to the picture (Filter>Noise>Add Noise). This simulates the grain that you see in a photographic print and helps everything blend together a bit more. Err on the side of too little grain, you want it barely noticeable (I used settings of 1.9% with Gaussian distribution).
