Tell the truth, your honesty should speak for itself, being able to speak fluently and clearly (not having to think about your answers etc...), good eye contact etc...
Be smart but also make sure you are comfortable, the last thing you need is to be unvcomfortable and fidgeting in Court, looking uncomfortable could give off a strange impression.
Make sure you have appropriate clothing available in case it is warm or cold in the court room for a similar reason. Similarly, have a drink available.
How is your temper? Remaining calm and consistent is also important, the latter should not be a problem if you are simply telling the truth.
Depending on what you are up for, getting agitated or showing a temper may be counterproductive, especially under cross-examination.
Have you gone through with your legal advisor the kind of questions which will come up and what the prosecution and their witnesses are likely to say?