As I see it, SAT prep has two main objectives:

1. Discover the most efficacious ways to solve common problem types.
2. Become proficient at recognizing opportunities to use those techniques in the wild.

It’s important that you devote equal time and effort to both. In other words, if you only take practice tests repeatedly, or if you only study techniques without ever taking a practice test, you’re doing it wrong. That’s why all smart tutors and good prep courses make you take full length tests in addition to showing you the important techniques. Because it’s one thing to be able to get a bunch of dangling modifier questions right when you know they’re coming. It’s another thing to have a dangling modifier jump right off the page for you when you’re taking the actual SAT and you’ll only see about two in a section.

If you’re going it alone, you should consider following a similar recipe to the one tutors and course use. Here’s a bare-bones SAT prep plan to help you go through this process:

1. Take a full length practice test. Ideally you’ve already taken a PSAT so you sorta know what’s up, but the real test is twice as long, and it’s best you know up front what it’s going to feel like to test for 3:40. There are 10 tests in the Blue Book, and you’re probably going to want to buy that, but you can also download one test for free from The College Board, which might be a nice way to get started. The goal here is to get a true baseline score for yourself, and to identify some obvious weak areas right away, so take it seriously: no long breaks, no cell phone, no Facebook.
2. Review the test. A lot. This is the hardest part for most people, but it’s one of the most important parts of the process. One of the major advantages of having a tutor or taking a good course is that they’ll force you to do this. Your goals here are to really understand your mistakes. Remember that the SAT is not subjective—there’s one correct answer and four incorrect answers to every question. Review the test until you feel like you could teach every question you got wrong to someone else. Take careful note of what kinds of questions (if any) flummox you so completely that you have no idea what’s going on at all.
3. Hit the techniques. The SAT has a whole plethora of different question types, and you’re going to need to be dangerous with the weapons that can dispatch all of them. This site, in general, is my attempt to spell out tips and techniques (math, writing, reading) that I think are best, but I know I’m not the only fish in the sea and I encourage you to do some research and take what you find useful from a bunch of different sources.
4. Take another practice test. Now that you’re good at solving plug-in questions when you know you’re looking at a plug-in question, it’s time to see if you can recognize what is and what is not a plug-in question when you’re taking a timed test. Be prepared to feel a little weird using some techniques, and don’t be discouraged if your score drops a bit. It’s the first time you’re taking a test in a new way. It probably won’t be smooth. Think of it like hitting live pitching after you’ve just gotten really good at hitting off a tee. Resist the urge to fall back on old methods when new techniques feel laborious. If you keep taking the test the same old way, you’ll keep getting the same old scores.
5. Review the test. Even more than last time. Now that the timer’s off, go back through the test and see whether any of your mistakes came from not recognizing opportunities, or from trying to apply techniques when it wasn’t possible to do so. Look at the questions you got right, too, if it felt like doing them took too long. Maybe there’s a faster way. Again, one of the great values of a tutor or course is that this is done for you with an expert eye. If you’re acting as your own tutor, this process will take a long time, but it’s also where all the big improvements happen. There will be times that you won’t want to review a test after you take it and score it (probably, you will feel that way every time) but if you want to improve, you have to do it.
6. Revisit the techniques. You won’t need to look at everything again, just the stuff that you didn’t know as well as you think you did when you took the test. Did you flub a few function questions? Brush up. If you have questions you don’t think you can answer yourself, ask an expert.
7. Repeat steps 4 through 7 until you’re happy with your scores. 

It might take a long time, and it will be incredibly hard work, but big improvements can happen. I see them every day.