Tell the truth. You must, you are under oath. However, do not depreciate yourself. If you don't know the answer or if you can't remember. Nobody is going to think you are stupid if you can't remember something.
Extra words create extra expense.
If you can answer a question with ten words, do not use one hundred. Don't volunteer information. You are not obligated to do so, and if you do, it could be damaging to your case. Just wait for the question, answer the question, and then wait for another question. Don't try to convince the other side of the merits of your case.