people who get good grades work to a set pattern, they schedule their work and their leisure and recreation time, giving them a good life balance and good grades.
Just because you have had poor grades in the past does not mean that you are destined to always have bad grades, you can pull this round, with a good plan a new strategy and some hard work and you can achieve the SAT results that you need.
If You have regular weekly / fortnightly tests, form study groups with your colleagues. This will really help you to fix the required knowledge in your mind, by the use of word association.
Have you ever tried creative visualisation? It is a great technique and involves kidding your subconscious mind into to believing the result you want is a foregone conclusion, so the subconscious actually helps to create the state of mind conducive to the grades that you need.
At the end of the day, you call the shots on your future, if you want to have a great time, and over do the drugs, and drink parties, do not be surprised if your grades suffer, remember though the effort you put in now will affect the rest of your life, so for a couple of short terms get your head down and dig in and get the grades you know you deserve.