Use SAT word lists found in many SAT study guides. Any words on this list that you cannot easily define and use in the appropriate context should be added to your personal study list. Incorporate these words into everyday conversations to become familiar with their appropriate context.
Add your own words into the blank spaces as you read the SAT sentence completion questions. Then look over the answer choices for any that resemble your own word.
Learn the logic behind the sentence completion questions. Complete understanding of the sentence meaning or vocabulary is not needed to do well on the SAT sentence completion questions.
Read all of the answers. Do not automatically choose the first "good" choice you see. More than one SAT sentence completion answer may seem like a possible "good" choice but there will only be one "best" choice.
Try out the first word in each of the answer choices for a SAT sentence completion question with more than one missing word. If the first word in an answer choice pair does not work, then you can eliminate that answer choice.
Answer practice sentence completion questions or take practice SAT tests. SAT practice tests will provide you with actual test-like conditions. Practice questions and tests can be found in SAT study guides and online.