You should ALWAYS be truthful in your deposition! The whole point of the legal system is to get to the truth! Perjury is a crime, and given that a deposition is sworn testimony, the laws of perjury applies during that proceding just as it would in front of a judge and jury. Moreover, untruths are usually found later on in discovery, and can really prejudice the deponent once found. Likewise, you may subject yourself to sanctions, including dismissal of the case or liability for the other side's attorney fees, if you intentionally lie during your testimony.
Listen to the question asked.
You must listen to the question - the entire question - that is asked. It is natural to be nervous during depositions. Nervousness often increases heart rate, blood pressure, and makes concentrating difficult. However, it will be presumed by everyone involved that you listened to the question, and answered the question that was asked. If you don't listent to the entire question, it can cause a significant issue down the line, as your answer may not be accurate, but you will nevertheless be held to it. Juries are very skeptical to people who change their testimony, even if it is for good cause.
Be sure you understand the question asked.
If you don't understand the question asked, don't answer it! Rather, ask that it be repeated (if you didn't hear it all), rephrased (if it was a poorly asked question), or paraphrase the question in your answer. For example, you can state something to the effect of, "If you're asking me what 2 plus 2 is, then my answer is 4." Everyone involved, even the lawyer asking you the questions, wants you to understand the question before you answer it so that they can rely on your answer down the line.
Only answer the question that is asked.
Generally, it is best to keep your answers brief if you can. If the question calls for a "yes" or "no" answer, the proper answer is "yes," "no," or "I don't know." Many attorneys simply have a checklist of questions they are working through, and long answers only makes the deposition longer, leading to mental fatigue.
Also, it is best to keep on topic. If you are asked where you graduated from high school, give the school name, then stop. The attorney does not want to know the history of your high school or of your education unless they ask for it. You run the risk of opening up other issues that wouldn't have been discussed had you kept your answers short. I'm certainly not suggesting that you hide anything - just keep on topic and to the point.
Of course, if the question calls for a long answer, such as, "Tell me every way your life has been affected due to your injuries," then let them have it. Your noneconomic damages settlement value rests on this testimony!
Generally, you are being called as a fact witness. Your deposition asks for facts - what did you see, smell, touch, taste, hear, or think at the applicable time. Don't guess. Guessing will get you nowhere. Juries are warned not to base their opinion on speculation, conjecture or guesswork. Thus, you shouldn't inject it in the case. If you are guessing, as opposed to giving an estimate, it means that you don't really know the answer to the question. Thus, the appropriate answer is, "I don't know." If you feel the need to answer any differently, let the other side know that you are guessing in your answer.
Rudeness will get you nowhere. Remember, the attorney is just asking you questions as a part of his or her job. He or she does not take it personally, nor should you, particularly in negligence cases. Moreover, you are being evaluated by the attorney to determine, among other things, your claim's settlement potential. The nicer and more courteous you are, the higher the claim's value. The more nasty and mean you are, the less the claim's value. Simple as that.
You want to make a good impression to increase settlement potential. Thus, dress for the occasion, like you are going to court. If you make a bad impression, you will deflate your claim's value. This goes also for any piercings - take them out to the extent possible. Cover the tattoos. The more professional you look, the better you will do.
Don't let the other side rattle you.
Some attorneys will try to push your buttons to see how you react. If you overreact to questions during deposition, they will likely assume you will do the same at trial. If you keep your cool, it will serve you well.
Don't show up under the influence of medication.
Do not show up to your deposition under the influence of any drug, if at all possible. This will help keep your mind sharp, will make it more likely that you are accurate, and will prevent the other side from asking to retake your deposition down the line. Of course, this is a general suggestion, as your specific situation may be different. Likewise, this suggestion pertains mainly to personal injury cases where the deponent is still treating.
Listen to your attorney!
If you are represented, listen to your attorney. He or she is trained in litigation procedure. They should know their evidence code and applicable deposition rules. They should know when to make an objection, and when not to do so. Should they object, listen to the objection. If the objection is "vague" or "ambiguous," that's a cue that it was a bad question that might need to be rephrased. Again, be sure you understand the question before you answer. If you don't, ask that it be rephrased. Also, the attorney may instruct you not to answer. If that's the case, don't answer.