Your "yes" will be easier for you to swallow if you place some necessary conditions on it. You might firmly state that only this one time will you agree. You might limit your involvement in time or responsibility. "I can help you this once, but only until 9 o'clock."

You may find yourself in a situation where you do not want to hurt someone. Your "yes" can be a kindness and a temporary fix until you can assess things. Paul wrote that we ought not to behave rudely, seek our own or think evil (1 Corinthians 13:5). So if you can't say no, learn how to use a limited "yes."

Further Reading:
"The Importance of Right Friendships" from Making Life Work. Online at www.thisistheway.org/booklets/LW/rightfriendships.htm