I have an L visa valid for 30 days. I want to get it extended for 30 days more. Please advise.

Well, to tell the truth, if you can extend it, the extended stay duration cannot be more than 30 days. You can apply for the extension at the nearby exit and entry management division of PSB. The registration form of temporary residency, travel itinerary, return ticket, hotel reservations or an invitation letter as well as your passport and application form are needed.