There are several skills and qualifications that are necessary and required to provide expert witness testimony. In my opinion there are a few skills and qualifications that I think are most important.
It is important to for and individual to have membership in professional associations of computer forensic examiners, formal training and certifications. This helps insure that the individual is properly trained and has the necessary skills to perform forensic analysis. This is critical to be a good export witness. This shows that a person has experience in the field and is qualified to do the necessary forensic work that can be used as an expert witness.
I think it is invaluable to have a thorough knowledge of subject matter and tools. The investigator must understand the kind of potential evidence they sought and analyzed, and understand the tools they used to gather and preserve evidence. In a court case it is important to be able to describe your process and the tools used in the process. As an export witness your ability do describe your process to provide effective evidence is crucial so that there is no doubt that the evidence is original and untempered with.
Finally it is important to maintain a positive reputation. As an expert witness it is important to be accurate, truthful and impartial. When performing the work as a forensic export it is crucial to be accurate and meticulous in your process. If you make a mistake it could potentially hinder the progression of a case. When reporting as an expert witness being truthful and impartial is also critical.