Psychological studies of "overlearning" have shown that for many purposes, there's not much benefit to drilling skills that you've already mastered. Once you're able to do something 100% of the time, there's no long-term upside to continued practice.
In fact, studying beyond that point of diminishing returns can be harmful, if it's distracting you from more important study tasks, or causing you to miss the forest for the trees.
Is overlearning a problem in SAT prep?
For the most part, no. The most recent studies show that while overlearning is not helpful for long-term retention, it can be useful when short-term retention is the goal (when you're studying for, say, one giant 3.5-hour test).
Besides, the most important things you can do to prepare for the SAT have a very high ceiling, and have great benefits beyond the test itself.
Still, there are a few areas where less is more.
Don't Overdo These
On some things, the point of diminishing returns kicks in pretty quickly, and you won't get anything at all out of pressing forward on them:
You don't need most of trig on the SAT. Learn SOHCAHTOA, the basic co-function identities, and the law of sines.
Same for geometry. Know the most important theorems and how to apply them, but don't worry about being able to prove them. On the SAT, it's enough just to know that say, alternate interior angles across parallel lines are congruent; you don't have to know why.
Vocab used to be really important, but that was de-emphasized in the 2016 overhaul, in favor of "words in context."
It still helps to have a somewhat sophisticated vocabulary, but to that end you'll get a lot more out of just reading some good books than trying to cram a list of five dollar words into your head.
As for test-taking tactics, a lot of the test prep giants want you to believe that they have a trove of "secrets," perfectly engineered keys that you can use to unlock the test without actually having to get better at the core skills.
Most of these "secrets" aren't secret at all. We're talking about things like process of elimination, trying out the answer choices, and reading the questions before you read the passages.
Yes, you can increase your score maybe 50+ points with basic test-taking skills like these skills that you'll learn easily for free with one web search.
But anything much more ingenious won't help, and can actually get in the way.
One workbook from a leading test prep company encourages students to practice looking for "trap" answers on each reading question.
Many students, especially smart ones, tend to overthink these problems already. Teaching you to play mind games with the questions will just make this psychological hurdle even harder to overcome.
You're really going to rule out an answer choice because it looks "too obvious" and must be a "trap?"
Sometimes, the answer choice seems obvious because it's right. Maybe you understood this passage more clearly or than others, or maybe you analyzed the question more accurately, or maybe it's just an easy question!
Get a few strategies and tactics under your belt, then get cracking on the content.
There are a few activities for which you can reach a point of mastery and even benefit in the short term from some continued practice after you've already mastered them:
Aim for perfect or near-perfect performance on these fundamentals. Once you can consistently finish test sections on time, remember your formulas, and avoid mechanical grammar errors, it's not going to hurt you to keep practicing, and might boost your retention in the short term.
Just don't let that practice get in the way of staying on top of your school work and outside reading.
As Much as You Can!
Here are some score-boosting habits that will always be worth doing more of:
Unless you already have a Ph.D. (and even then perhaps), you're always going to have room to grow in these areas. Keep pushing your reading up a notch.
To get you moving, I've started a list of books that I think will challenge and surprise you. I plan to keep adding to the list, so check back if you start to run out of material.
I'll talk more soon about how to approach each of these activities. For now, just get going and read something!