Blend textures and materials. Too much of one thing is never a good idea. If you find a table that you like with a stone top, balance the table with warm wood chairs and top it with flowers in a metal or ceramic vase.
Place objects on surfaces around the room that catch and reflect light.
Do not start collections. And if you do, display them all in one area of the room. Collections spread around a room create chaos for the eyes. Who wants to walk into a room full of porcelain frogs or shot glasses?
Frame your art alike. It makes your space (especially a small space) flow and creates continuity.
Keep it simple and do things in groups of three: three colors, three objects, three types of material. You will be pleasantly surprised by the result.
If you love bright color, use it in objects, lamps, and small accents in the room, and keep it to three colors in each room.