Say your raw scores on Test 2 in the Blue Book looked like this:
That'd give you 650's across the board (assuming an 8 essay). Now say you made one fewer multiple choice error in the 3 reading, 3 math, and 2 writing sections. That's going to add points directly because of the correct answers, and it's also going to erase penalties to the tune of 0.25 per answer.
CR: 53 + 3 + 0.75
M: 43 + 3 + 0.75
W: 42 + 2 + 0.50
Your new raw scores round up, so now they look like this:
Your score just went from 650 + 650 + 650 = 1950 to 690 + 690 + 690 = 2070. Get some.
Of course, the actual improvement you'll see could vary a bit based on where you're starting, but 100 points is a pretty good estimate for this kind of raw score improvement.
What does this mean for you? It means that if you can identify few weaknesses that appear often on the SAT. Say you struggle with dangling modifiers, main idea questions, and questions that can be solved by plugging in. If you use focused practice on those kinds of questions until you're a pro, you can see a big improvement, even if it's late in the game.
It also means that although it's obviously prudent to practice by taking whole tests, it's also a good idea once in a while to buckle down with a drill that contains only the specific kinds of questions you're looking focus on. One of the advantages of taking a good prep course or working with a tutor is that an experienced tutor or prep course teacher should, with all of her familiarity with the test, be very quick to help you identify your weak areas, and provide drills tailored to your weaknesses. If you're studying by yourself (as most of my readers are), I'm trying to help you out by creating these drills built to identify weaknesses. They have answer keys that link directly to salient techniques to help you with the questions you missed, and to provide more focused practice with those kinds of questions. It's a work in progress, of course, but I think what's there is good and what's coming will only make it better.
The bottom line: If your score isn't where you want it to be yet, don't freak out, and don't try to do everything at once. You're not going to jump from 1800 to 2100 (or 2100 to 2300, or 1400 to 1600) in one test. But if you can identify a few weaknesses every time you take a test, and you're able to make sure you don't make those kinds of mistakes going forward, you can eventually see a huge improvement. Look at your practice mistakes closely to try to identify patterns, seek focused practice based on your personal weak areas, and set reasonable goals to improve your performance on those specific things. If you do so diligently, you'll be happy with the results.