Weigh up the pros and cons
There are situations where saying 'no' isn't on: when a workmate with serious personal problems asks you to give her a helping hand, better to say 'yes!' But when someone asks to lend them money and you're not happy with it, then refuse. Just weigh things up before you reply and your instinct should guide your decision.
Take your time
When we're put on the spot we often feel trapped and end up accepting an unwanted invite or extra work. Why give the wrong answer straight away? Ask for a moment to think before you give your answer.
Don't feel guilty
If someone asks you a question, by nature it's only logical that the answer isn't a guaranteed 'yes.' You're well within your rights to refuse to lend your friend £300 or to turn down dinner at your parents'. It doesn't make you a bad person! The people who matter understand that you can't always be there, and they won't hate you for it.
Train yourself to say 'no.' Practise how you'll formulate your sentence in those situations where you're screaming 'no' but you feel obliged to say 'yes': "I'm really sorry, I can't," or "Sorry, I won't be able to." Say them over in your head so that you'll be ready when the time comes. When you're in the situation, tell yourself things like: "I have the right to say what I think," "I'm not going to put myself on the line just to make him/her happy" and "I'm not at everyone's beck and call all the time."
Be tactful when you give a negative answer. If someone asks you a favour, don't just tell them to go and take a running jump: explain why you're sorry but you can't help. A bit of empathy and diplomacy can go a long way and make 'no' easier to accept.
Suggest a solution
If it really pains you to say 'no' and you know you'll feel guilty, suggest an alternative: "I can't look after your kids on Saturday, but I can do Tuesday if you like." Your willingness to help won't go unnoticed.