Look for the following:
Pronoun errors: (agreement, reference, and case)
Modifier errors: (agreement, reference)
Comparison errors: (agreement, reference, parallelism, "apple to apples")
Verb tense errors: (correct tense, correct relative tense)
Subject/Verb Agreement (sing/plural)
Idiomatic forms/Use of wrong words
Other: (punctuation, redundacy, style, active/passive, conciseness, etc).
You need to create a system to go through the above list.
My system varies from sentence to sentence but for a complex sentence I always look for parallel structure first, then agreement, reference, tense, then the rests...
Sentence correction lends itself well to POE.
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993