Use pieces that are within 5 inches of each other in height. They will appear to harmonize with each other, even if they are in different styles. You can also use pieces with similar styles of arms, whether rolled, straight, or nonexistent, but height is the most important factor.
Switch the pillows that originally came with the furniture. That is, if your loveseat is blue and your sofa is white, put the blue pillows on the sofa and the white ones on the loveseat. This will give the two pieces something in common with each other and harmonize the room.
Drape matching throws over the back of each piece of furniture; this will tie the two pieces together.
Use matching slipcovers on both pieces to make them coordinate exactly.