Yes, for most of us this is true: "Saying no is difficult. For some, it's nearly impossible. But sometimes it's crucial to your success."
If you find yourself saying yes too often, use these three tips to prepare to say no:
1. Set intentions. Often we don't say no because we're not sure what we're working toward. Take the time to write down what you want to achieve and what will help you get there.
2. Prioritize commitments. Make a list of all your current commitments and prioritize them. Commitments that are low on the list should be "no" items.
3. Make no your default answer. Assume that you will say no to any new requests that come in, unless they meet a short set of criteria. Will the project help you grow professionally or personally? Does it fit into your intentions for the year?
Source: 3 Tips for Saying Yes to No - Management Tip of the Day - February 24, 2010 - Harvard Business Review