Sentence Correction

Hello,

I've been preparing for the GMAT which I plan to take early next year. I used to think that SC is one of my strong areas and I need little practice but to my surprise and dismay I did not score really well on the GMATPrep Software. I have completed the theoretical things but given the time frame its really difficult to apply all those rules and come out on top. Sometimes I face a difficulty in recalling the rule itself.. I really need inputs on how to go about SC and what critical things to focus on

Thanks