"Does test preparation help improve student performance on the SAT and ACT? For students that have taken the test before and would like to boost their scores, coaching seems to help, but by a rather small amount. After controlling for group differences, the average coaching boost on the math section of the SAT is 14 to 15 points. The boost is smaller on the verbal section of the test, just 6 to 8 points. The combined effect of coaching on the SAT for the NELS sample is about 20 points. The effect of coaching is similar on comparable sections of the ACT. The average score increase on the ACT math section probably lies within the range of 0 to .4 points, while the coaching effect on the English section is about .3 to .6 points"
[1] http://nepc.colorado.edu/files/Briggs_Theeffectofadmissionst...
Most likely, a correlation is not causation thing -- people for whom success on the SAT is most important are likely to spend more money on test prep, but also be best generally prepared. As ueqirat points out, the research that has been done on SAT test prep utility hasn't actually born out a real effect.
The secret to standardized testing is fast reading comprehension. I've always been a fast reader (without sacrificing comprehension). It frees up time to go back and really spend time on the tougher questions that just isn't available if you have to spend a long time just reading the question (it's more than just in the reading comprehension section, since you have to actually read every question to get through the test).
It's basically "Write a generic 5 paragraph essay using memorized argument flow."