When shopping for accessories (Or making them, if you are a DIY kind of person), here are few points to consider;
Colour
Ensure the pieces complement the colour scheme in your space; If all the pieces you pick are so different, you may end up with a cluttered look that is visually distracting.
Consider the size of your space and buy things that are in scale to it. If your room is small, avoid very large pieces that may fill up the room and make the place seem cramped. If you want to use large art works ensure the paintings creates visual depth. An example would be a picture of a landscape, a seascape or murals of cityscapes.
Accessories should reinforce the theme of your house or space. If for instance you are working with a classic theme, the paintings that have a reinaissance feel should be adopted, the area rugs should have the classic motifs and prints. If you use different accesories, the theme you sought to achieve may not be realised. There are exception to this rule especially when you are fusing two styles.
Go easy on the number of accessories you place. You dont want your house coming of as a curio shop or a gallery. Consider the principles of design when placing them and dont be afraid to remove any exceeses. If you are a collector, find neat ways of placing your much loved pieces without cluttering your space. You can as an example, make a collage of your different paintings on one wall, like the stair wall or dinining wall.