First, heres how the scoring works:
Blank: 0 points added
Wrong Answer: 1/4 point subtracted
Correct Answer: 1 point added
So, does this mean you should leave some blank? For most people, probably. I wouldnt even attempt to do some of the harder problems on the test (the 16-20's, end of section stuff), but instead concentrate on the ones I do know and make sure I get them all right.
Second, the thing that will help you most on the test is knowing the vocabulary. However, theres no real hope of you learning it all between now and Saturday, you should have started practicing two months ago to really know the words and definitions. Best hints I can give you overall would be to pace yourself and dont feel like you have to answer every question. Focus on eliminating answers. SAT answers (especially in reading/writing sections) are terrible: for the most part, you have to chose the answer thats 'least wrong'. Crossing off the obviously wrong answers will help narrow your search.
Good luck!