I feel like my good hearted nature is being taken advantage of lately. I can't say no when someone ask for help and all I ask for in return is honesty in our friendship. Without going into details how have people handled others taking advantage of their good nature?
Very badly. I have no natural tact at all and try to get by on simple truth as I know it and openness. It is just about always understood as weakness and I'm convinced now that it is my fault for expecting people to behave in a way not natural to most humans. I would change if I could because I don't have the impetus Christianity used to give me to persevere. Nowadays I keep the same M.O. because I simply don't know any other way. It's a world of complexity I don't want to enter.
Say NO and stick with it. Some people will ask incessantly in an attempt to wear you down. Saying yes at this point is the worst as it is intermittent reinforcement and is the most difficult behavior to break. If the person you are dealing with is deceitful (you mention honesty) - re-evaluate the friendship.