Tips for Adding People into Perspectives

1. Scale. 
It can be difficult to get your people textures at the right scale in your architectural illustration. If you're not good a doing this by eye, then a good trick is to insert a crowd of lowpoly 3d people into your scene and do a quick render. You can then use this image in a separate layer in photoshop while you're compositing your people. (or trees, cars etc.) 
2. Context. 
Does the person fit the context of your image. Its self-explanatory you're not going to put a man in jeans and a T-shirt into a formal business environment, but you should also review the persons pose, and whether they fit naturally in the location you're trying to put them. 
3. Lighting.
Compare the lighting in your illustration with the lighting of your people. For example, people textures that are sharply lit will obviously fit better into a sunlit area of your illustration, but take note of the direction that the light is coming from as well. 
4. Color Harmony.
I believe colour harmony is important when compositing people with an architectural illustration. What I mean is that the colour of the clothing of your people should be in harmony with the colours in your rendering. This is important because what you're trying to show off is the architecture - not the people. If your people textures represent a psychedelic rainbow of colours, then that could distract the viewers eye somewhat ! 
5. Colour Correction.
Not the same as colour harmony, but has a similar intention. Your people textures may have a red hue to them, but your architectural illustration has alot of blue tones in it. For a more convincing result its good to colour correct your composited texture to match the colour tones of your image. The same comment is valid for the people texture's saturation. Not only is the result more convincing, but it also means that your people will not stand out, and detract the viewers attention from the architecture. 
6. Blending Modes & Transparency.
Once you've pasted your people into a scene, don't just be happy to leave them in their original state. Play around with the layer blending modes, change their transparency, blur them, even make a clone of the layer and apply different blending to each layer. If you just leave them in their raw state, most of the time they won't look like they blend with your image. Playing around with the layer modes etc., can help create a better harmony with your architectural illustration. 
7. Composition in Space.
Should that person really be there ? Make a judgement on the composition of your people within the scene. Move them around until they look right, and so that they don't diminish an important architectural aspect of the view. 
8. Shadows and Reflections.
Pretty straight forward, but don't forget to create shadows and reflections of your people textures if necessary - it helps themsit better in a scene.... You can also blur shadows or reflections if the scene requires


From http://www.3dfabrique.com/architectural_illustration.html