1. Make it concise. The fewer words used to make a point, the better. Go back and cut, cut, cut—to the bone—keeping only those lines that convey character and thus entertain.
2. Include subtext. What people say and what they really mean are often different things, with meaning often buried under the lines, and not floating on them.
3. Be illogical. People are, especially when they speak—especially when they argue. For this reason dialogue shouldn't always make sense. Nor do people listen when others speak; even if they do, they often don't respond directly to what's just been said. When writing dialogue, try, every three or four lines at least, to deal from the bottom of the deck, to have characters say something surprising if not bizarre, something at least not obvious.