This simple negative is the hardest of the lot because you probably feel compelled to give a reason that gets you off the hook, but sometimes - especially with people who are inclined to override your excuses - it's the most effective way of turning them down. Don't think too much about it, and don't try to fill in the silence with mumbled excuses. Say the words, and then keep quiet. You'll be amazed at how effective this simple assertion can be.