Blizzard has never revealed their system for premotion/demotion -- but basically you need to play some ladder games and have a win percentage that is good against people in the league you are in.
That whole thing is a real mystery at the moment, if anyone can shed some light on it fantastic... but failing that, just keep winning is the only answer.
keep winning till your mmr fits that of silver league players or higher(check their stats after each game) . then lose some, win some and tada, you will get promoted ;-)