Whether you are declining a party invitation or refusing an additional task at work, there are some simple ways of learning social etiquette like how to say no that satisfy social etiquette.
When you are asked by friends and family to participate in some time-consuming project or an acquaintance keeps asking you to get together sometime, you can offer up how busy you are as a way to say no. You don't need to go into detail, just stick to polite conversation. Just say something like, "I'm so overbooked these next few weeks, that I really can't. However, I'll call you when things lighten up." If they insist, keep insisting right back with something like, "I'm sure you understand where I'm coming from."
When you need to turn down an invitation for lunch, dinner, play date or a party, be direct but polite. Say, "That sounds fabulous, but I'm just not going to be able to make it. How about I call you when I find a free day?" Another way to say no, especially if you really do want to get together but just can't for a while, is to be honest, "I love spending time with you, but I am spread so thin right now. I need to get a few things behind me, then I'll be much better company."
When your boss or another co-worker wish to add yet another task to your already busy day, determine whether or not it is an opportunity worth pursuing. If it is, then you need to extract yourself from other obligations gracefully. However, if it is just busy work or picking up the slack for another, social etiquette dictates how to politely say say no. The best way is to be direct and say, "With my current job responsibilities, I just can't commit to that." Be diplomatic and say, "I'm just not sure that task is suited to my strengths. No thank you." Simply saying, "I can't right now, but thanks for considering me on it" should suffice as well. Be firm and polite in all social conversations and soon you'll find that saying no gets much easier with practice.