You have a right to say no. Remember that others may take you for granted and even lose respect for you if you don't.
We say "yes"to others because we want to please them. But when eventually we can't continue, we let them down and we feel guilty. Both parties suffer. Recognize that a desire to please often prevents us from saying no.
Be polite, but firm in saying no. You only build false hopes with wishy-washy responses. For instance, the phrase "I'll try to be there" in response to a party invitation is giving yourself an excuse to avoid a commitment. It doesn't do anyone any favours.
Some experts recommend keeping your answer short. This way, you can say no without feeling the need for a lengthy justification. ("I'm sorry, I'm not available that night.") On the other hand, others say that giving a longer answer with reasons reinforces your credibility. Let the situation decide.
When in doubt, it's easier to say no now, then change your mind to a yes later, rather than the other way around.
Já que é uma das minhas premissas de 2009: ter "juizo"...