One of my favorite ways to add character to a room is to mix two or more design styles together. There is nothing better then seeing a traditional chandelier hanging over a modern seating area. Not only does it add a layer of sophistication, but it gives you many opportunities to add your own personality to the design.
Go with what you are Attracted to: Achieving a well done eclectic space does require some knowledge...not rules, just some general guidelines. Many people are drawn to a certain commonality, whether this is color, lines, or scale. This is where you can begin your design for cohesive harmony. You may be drawn to the lines of a traditional wing back chair. Why not take those lines and find a modern fabric that coordinates with them and upholster the chair in it?!
Create Balance: Finding the "balance" is key. The best way to have your eclectic space look intentional is to give more of one style and take less of the other. Try for a 80/20 or 70/30, 80% of one style and 20% of the other. One of my favorite tricks to achieve the eclectic look is to make all the lighting fixtures and rugs traditional and all other elements modern, or vice verse. This gives the correct amount of balance and looks beautiful.
Consider the Elements: Before putting those five pillows together consider what they do to one another when put side-by-side. By throwing all these pillows together it could look very "hodgepodge", but finding one element that works with all five elimantes this issue. This could be one color found throughout the pillows or even the scale of the patterns. This "guideline" applies to furniture as well.
Art: Art is one of the hardest parts of the room for some people to envision. My rule is: GO FOR IT!! Art does NOT need to match your room. If you like it and the proportion works, use it. Art is meant to be seen not blend in.
The art of mixing old and new and having it work together but in the same token be completely different, can become confusing, but, when done well, it can be breathtaking.