The goal of the rebuttal is to clarify major misunderstandings by the reviewers and incorrect statements in the reviews. It also serves as a starting point for the discussion among the reviewers and the area chairs.
An effective rebuttal focuses on major points. It is not helpful to try to address every criticism in the reviews. By prioritizing and focusing on the major concerns, you are demonstrating to the reviewers and the area chair that you understand the high level message in the reviews.
An effective rebuttal addresses reviewers' criticisms by explaining where in the paper you already address raised point and how. Another solution is to explain your line of thinking that would clarify the issue even if it was not included in the paper.
It is not helpful to promise to expand your paper to address all the questions raised by the reviewers. Everyone knows you don't have room to add to the paper, and they won't take your promises seriously.
A good rebuttal is polite. Accusing reviewers of being ignorant or petty does not get them excited about the paper.
It is useful to summarize or rephrase the criticism before you address it. While the room for rebuttal is limited, this is an effective way to let the reviewers and the area chairs know that you understood their concerns and have intelligent answers to the questions raised in the reviews. Using complicated acronyms and numbering systems to encrypt which comment you are addressing (for example, R3C4 for comment #4 by reviewer #3) usually gets everyone frustrated when they have to go back and forth between the reviews and your rebuttal. Frustrated reviewers are less likely to take your rebuttal seriously.