Another rule of thumb for grouping or clustering objects is to stick with odd numbers. For example if you are grouping candlesticks on a mantle, try using one tall, one medium, and one short for a more interesting look. Low baskets or trays are a great way to display a bunch of dissimilar items together and make them cohesive and feel special.
Some of my favorite accessories to use in a room are books, they are relatively inexpensive and a great way to incorporate a clients personality and preferences. You always want to give the appearance of books collected over time, so stay away from color coordinating the spines. Instead alternate the colors for a richer look. Keep in mind that the arrangement is more pleasing to the eye when the books are not lined up on the shelves. Try standing some up, while laying a few down, adding frames and other objects to give a more personal feel instead of a "library" look.
Candles are a great way to bring in some lighting and ambience in the room. Try using scented candles but stick to whites and creams so they are not demanding your attention. Display them in hurricane shades or clustering them on a tray in different heights instead of scattering them about the room. There are great options on the market for wickless candles so you never have to worry about them being a fired hazard.
Fresh flowers are a must in any room, just a simple arrangement of calla lilies is enough to liven up a room.