Make use of the high-frequency GRE word list found in many GRE study guides. If there are words on this list that you cannot easily define and use in the appropriate context, add them to your personal study list. Use these words in your everyday conversation and writing to become familiar with their proper context.
Insert your own words into the blank spaces of a GRE sentence completion question. Then look at the answer choices for the one that is closest to your own word.
Read through each of the answers. Don't automatically select the first "good" choice you see. The GRE sentence completion answers may offer two possible "good" choices but only one "best" choice.
Try out the first word in each of the answer choices for a multiple fill-in-the-blank GRE sentence completion question. If the first word does not work, then you can eliminate that answer choice. This will save you time and reduce any additional stress.
Watch for metaphors. If the GRE sentence completion question contains a metaphor, then that metaphor will dictate the word choice. Pick the answer that best fits the sentence and the metaphor.
Practice. GRE practice tests and sample questions will give you the opportunity to become familiar with the sentence completion question type.