Write down everything you can think of. Do this over several days. Later you can edit and rearrange your thoughts into some kind of logical order. Read it over to yourself until you can almost recite it by memory. This will help you overcome your initial nervousness on the stand. You won't have to search for the answers or what order you are going to list your own qualifications. It will just come out. This same "bio" can become your affiant statement for future gang warrants.
Go out and look at your local gang graffiti. Know what the common abbreviations and symbols mean. ATM does not mean automatic teller machine to a Hispanic gang member. It means "A Todo Madre!" (Everything mother). Recently I saw a local gang cop on the stand in court who did not know what the Calo expression "Rifa!" meant. This word is taken from the old Calo word for marijuana, "griffa," and which the word "reefer" comes from. Rifa is a very common exclamation for something good. Totally clueless to this expression, his credibility in front of the jury was lost and after a few more poorly answered questions the judge stopped the testimony and dropped the gang allegations.
Dress to impress.
Another mistake many experienced gang cops make is to wear their plain clothes or working clothes to court. Like the grizzly veteran cop described earlier, the boots or tennis shoes, green nylon raid jacket, and pistol hanging on the hip do nothing to impress the judge or jury. If you are expecting to be qualified as an expert, look the part. Wear a conservative dark colored suit with a business-like shirt and tie. If called in from the field to testify, a uniform is preferable to street clothes. If you must wear jeans and a T-shirt, have the prosecutor apologize to the court and tell them that you were working when summoned to court.
I must compliment the officers of the Los Angeles Police Department. They must be required to attend some class on courtroom appearance, which must be skipped at the Los Angeles Sheriff's Academy. I have spent many a day sitting in a downtown Los Angeles courtroom, and on average, whether in uniform or suits, the LAPD officers look neater and more professional than the LASD deputies. They may be no better at qualifying as an expert and testifying in general, but at least they look like they do.