Mixing it up in the right way and avoiding the wrong way.
The difference between a nice room and a well put together and interesting interior designed room is often in the details.
Textures play an important role in creating the right mood. Smooth and shiny for sleek minimalist moods and rough, matt and rustic textures if you are trying to achieve a relaxed casual mood.
If everything matches it becomes boring and uninteresting, aim to add some contrast in textures but without going over the top. For example if everything is shiny, marble floors, glass tables, high gloss furniture and satin fabrics it can be too much, so by using some matt finishes which could be in a rug, fabrics used and natural wood we are adding interest without bold colours or patterns.
Speaking of patterns, of course this is the simplest way to inject interest into a room. However be careful not to add too many different patterns as this can become confusing and overstimulating. A maximum of 3 or 4 is a good rule to follow and try to keep them within the same colour code.