Being voiced marks you as a particularly good user. It grants you a position at the top of the userlist, the ability to talk in moderated chat, and shows that you are a respected and helpful chat member. While obeying the following will not guarantee you voice status, it's a good start and will certainly improve your image in the staff team's eyes.

Be helpful to others. Not everyone who uses Pokémon Showdown is an experienced battler, and answering their questions or otherwise helping them will gain respect from other users.
Tolerate newcomers to Smogon and competitive Pokemon. The majority of new users have not played competitive Pokemon before and may even be in an internet chat room for the first time. As such, treat them with respect, help them if needed, and make Pokémon Showdown a better place for them.
Respect authority. While you might not agree with the decisions of staff members at times, please do not question or otherwise call out their decisions. If you feel that a staff member has made a wrong decision or abused their power, please PM higher ranked staff in a polite manner.
Be a generally positive presence in the chat. Answering questions, helping users, and contributing intelligently and respectfully to discussions is a surefire way to get voice.