People often complain about pushy coworkers, grouchy spouses and having a jerk for a boss, but often this can be the result of a lack of assertiveness. Essentially you want to be able to tell people what you think without putting anyone off or insulting them.
This means not using aggression or hostility and to respect the other person's right to interact in the conversation. Assertiveness is milder than aggression; it is standing up for your rights and expressing your thoughts, feelings and beliefs in a direct, honest and appropriate manner.
Being assertive is a skill that you learn and you will find the quality of your relationships with others, will improve as you grow more and more assertive.
To start you will want to master Basic assertions such as "No thank you, I don't want any more cake" or "No, I didn't like that movie". When you feel comfortable expressing yourself using basic assertions you can then start to learn how to use Empathic assertions. In Empathic assertions you make it clear to the other person that you understand their point of view but you are not going along with it. "I understand that you want me to go with you to see the show, because you don't want to go alone, but I don't feel well and do not want to go out."
When you encounter pushy people, you start with a basic assertion and when they push harder you move on to Escalating assertiveness. The conversation may go something like this;
Pushy Person: "Have some more cake."
Assertive Person: "No, thank you, it was delicious, but one piece is enough for me."
Pushy Person: "I made it especially for you, please have some more."
Assertive Person "No, thank you."
Pushy Person: "Come on, it won't kill you."
Assertive Person: "For the third time, no thank you. Please don't ask me again."
Another good tactic to use, especially when dealing with manipulative people, is to use "I language". For example, instead of saying something like "You are not being fair" say “I feel that you are not being fair". They may be able to deny that they are being unfair but they will not be able to deny your feelings.