A few thoughts on this subject!

Start out with a model that has accurately scaled components. Door and window frames, window muntins should be properly scaled. Its all in the details! and accurate details.

When modeling, dont guess at sizes and deminsions of items, measure and build like you are a carpenter

Edges of items should pretty much always have a bevel or radius to them, even if so small its nearly undetectable, hard 1 line edges will make items look toyish. Light needs to be able to flow around that edge, your brain will know the difference.

Texture colors are often too saturated, this too will create a toy like appearance.

Reduce color saturation on textures somewhat. Make sure textures are properly scaled and applied.

Study the work of artist at sights that are more geared to the professional, pixelogic and CG society.

Read study and learn about photography, you will learn concepts that are very usefull for CG work