Though I've gotten better over the years, I still struggle with this one word. Seems (at first, at least) much easier to say yes. I'm talking about the small favors that people ask you to do, the ones that you're sure will only take a few minutes of your time. The most insidious: the favors that you know would be fun to do, or the ones that are almost work-related.
Be wary of saying yes to too many of these. Inevitably, they'll add up until WHOA, suddenly you find yourself spending more time on these quickie favors for other people than you are on your own work.
Stay focused on your goals. Prioritize.
Learn how to say no politely but firmly without feeling guilty.
To clarify (because I'm already anticipating at least one person piping up and righteously saying that she's GLAD to want to say yes to those asking for help): I'm not saying you should ALWAYS say no...just be selective and be realistic. And be able to say no when you need to.