Improving your vocabulary is the best way to improve your score on the sentence completion questions. This can be challenging, but fortunately the SAT uses some words repeatedly. Most SAT prep books and courses will provide you with a list of the vocabulary words appearing most frequently on the SAT; focus on learning these words. Using flash cards, writing out sentences with the words or making your own crossword puzzles using online software can help you improve your knowledge of the most common SAT words and thereby improve your SAT score.
When you are moving through the sentence completion questions, predict the word that you think will go in the blank before looking at the answers. If you have a good idea of the word you are looking for, you won't be inclined to get tricked by wrong answers. This will also help if you do not know some of the vocabulary words in the answer choices. You can decide if that word has a positive connotation or a negative one and then look at the words you don't know to determine whether they sound positive or negative.
Skim long passages first rather than reading them carefully. Read the whole first paragraph, the whole last paragraph and the first and last sentence of each middle paragraph in order to get the main idea, the topic and the scope of the passage. Then answer those questions referring to a specific line in the text. Go back and read the line in question, as well as a few lines before and after it. You will probably end up reading the entire passage before you get to the general questions, but you will do it in a more focused way, allowing you to retain more. You also won't waste time by reading and then re-reading.
Sometimes, students do not finish the entire critical reading passage section. Choose the passages you find the easiest to deal with first, in case you don't finish the section. That way, you will be spending more time doing questions on a subject you are comfortable with, such as literature or fiction passages, for example. If you run out of time, the passage you struggled with -- and the one that you were least likely to get questions right on -- will be the one that doesn't get finished.