Assume you will have to substantiate EVERY word you say.
Preparation is key - read all relevant documents.
Raise all points, good and bad, with your counsel BEFORE the inquiry.Always acknowledge any weakness in your case, particularly in your written proof of evidence.
When writing your proof of evidence, make sure you get the introduction right - this is the most important section.
Agree as many facts as possible with opposing parties in advance.
If an opposing expert has reached very different conclusions from the same evidence, try to understand why.
When part of a team of experts, establish each individual's scope of evidence in advance.
When giving evidence, MAKE SURE YOU CAN BE HEARD - if possible, check acoustics/ microphones in advance.
Look alert, avoid fidgeting, try to address your answers to the inspector, not the barrister asking the questions.