Master the art of working with perspective
Analyse where the light is in the picture, add a new layer of 20% Opacity, and brush soft, black colour to shade opposite the light source. This keeps things realistic.
Cmd/right-click to Transform and Cmd/right-click again to choose Perspective Transform. Reshape two-dimensional images to fit a new perspective; great for placing an image onto a wall, for example.
The Tilt Shift blur filter can be applied to cityscapes to give focus to tiny subjects, giving a toy-town effect.
The Photo Filter and Curves adjustments both add colour and light to a picture, making a pasted subject look part of the background.
Be careful where your subject is standing: nothing breaks the realism of a composition like a badly integrated foreground. Make sure that buildings or trees on the floor are visible around your subjects’ feet (or paws in the example above).
When adding subjects of different sizes into an image, it is absolutely imperative that the edges are perfect, as it will become immediately obvious otherwise. Use the Refine Edge tool and gently erase any hard subject edges.
Dodge and Burn are vital tools in any composition, big or small, but when you’re working with strange perspectives, they can help to balance and enhance the light and shade in a picture.
Make sure that the angle of the subject matches the angle of the background. Pictures shot from similar positions are much easier to edit together, no matter what the size or scale of them is, so choose your subjects and backgrounds wisely before you start. A bit of prep will save hours of editing.
The colours of pasted subjects need to match the environment they are in. Match the colours either by going to Image>Adjustments>Replace Color or by creating a new layer, setting it to Color and brushing over the top.
Once you have made all of the major edits and colour corrections, a nice lighting effect can finish off a piece. Despite a reputation as being a bit naff, the Lens Flare is a popular choice, as is the Point Light or the Infinite Light option, which can create a light source in a corner of the picture.