They do a bonus system. To get this allyou have to do it upvote thier posts. The bonus is 0-100% of your shares.
They calculate this bonus on a 28 day running total. So if you upvote 28 times in 28 days you get 100%. The best way to do this is to make sure you upvote thier post or comment everyday. This will double your shares. Then you can save up your steem to get more shares or enter the competitions that give away free sbi shares to the winners.