I Joined fiverr one year ago and started earning from my hobby of web and graphic designing besides doing my tough Banking job. I started spending 2 hours daily on fiverr. After one year I am now level 2 seller and fiverr is now my major second source of income. What i learned as a successful seller, you must follow these tips:
Never say no to buyers: Never take any order, which you can not do and if you get the order, no matter how hard it is for you to do any job never say NO to your buyers.
Give unlimited Revisions to buyers: Always communicate politely and give revisions till satisfactions of the buyer.
Work after completion of the order: You must give revision and work on an order even if it is marked as completed, this will help you get more work from existing buyer.