Open shelves or shelving units can become a dumping ground for untidy piles of books, magazines and unfiled bits of paper. Make them more of a display unit than a piece of functional furniture and you'll smarten up your space and have an impetus to keep the shelves tidy.
One instant neatening trick is to rearrange your books so the spines are grouped into colour categories (blue spines together, green spines together etc) and in order of descending size. It sounds a bit of a faff, but it does make a difference.
If the shelf is housing something more useful than beautiful, hide it away in a good-looking box or container. If you can't find something shop-bought you like, cover a cheap, plain box file, magazine file or shoebox with a thin piece of printed cotton fabric using some spray glue. Glue two 15cm strips of ribbon to the outside of the top and side of the box (underneath the fabric) to keep the lid closed.
To fill some empty space on a shelf, think in threes. Line up three small plants in matching pots, three framed prints or three coloured glass vases/candle holders and dead wall space will instead be a point of interest in the room.