Recently read an article on the importance of filtering out things that come our way and saying ‘No’ to almost 90 percent of them. This helps us focus our energies on the most important ones and excel at them.
Reproducing a useful quote from the article:
Every successful businessperson has to learn how to say no,” he told me that day. If you spend your life in business, you will see dozens or perhaps hundreds of potential deals. A small number will be highly attractive; most will be average or below average. The path to superior results is to accept only the best ideas — indeed, no venture capitalist or merchant banker could survive for very long without saying no to 90 per cent (or more) of the pitches he sees.
You can be diplomatic, firm, or a combination of the two, but you must be comfortable with the idea of handing out rejection.