I have tried Manhattan's SC guide and its good one (no practice as most questions from OG)
try also GMATCLub's SC Book (plenty of practice)
I heard Aristrole SC is good for practice but I suggest using the above two as reference and becoming super active on SC section of the forum.. if still no improvement try egmat
SC course... I hear its good esp for non natives
All men dream - but not equally. Those who dream by night, in the dusty recesses of their minds, wake in the day to find that it was vanity... But the dreamers of the day are dangerous men, for they may act their dream with open eyes, to make it possible.