Saying NO often can be very hard. So often we do things even if we do not want to do them specially to meet the expectations of everyone. But doing things against your will and possibilities will end up in feeling resentful. Prevention is always better than cure. So to protect yourself from feeling dreadfulness you just need to learn to say NO.
A simple and direct NO is the best solution for many upcoming troubles.
Most common ways of putting a NO:
I can´t do this right now.
NO, thanks. Thank you for asking.
Sorry, but NO.
Assertive ways of putting a NO:
Start the sentence with the word NO
Use nonverbal ways like nodding your head as to state your NO.
When someone makes a request it is wise to ask for time to think it over because it is difficult to say NO immediately. No matter how decisive the situation is you always have the right to say "No". So to have the power of denials just strengthen your will power a bit more.