The only bad way to prepare for standardized tests is not to prepare. You don't need to work with a tutor for three hours a day between now and when you take the test, but you do need to do something.
Reading is crucial to test prep
The simple, most powerful way to prepare for the SAT, ACT, or PSAT is to read. A lot. With a dictionary. If you love to read and develop your vocabulary, then you've got a head start. If you dislike reading and avoid it at all costs, then you're going to need to develop some conscious strategies as early as possible in order to build your skills and prepare for exams that are heavily weighted toward verbal skills.
Beyond that, you need to think about your particular needs.
The self-study test preparation option
It might help you to think about test prep along a continuum, from least to most intensive. The less intensive (and less expensive) approach is to work on your own with one or more prep guides, perhaps in conjunction with some online resources. This requires a lot of self-direction and motivation.
You'll want to plan on 20 to 30 minutes every day or so, for several months in advance of a test date. You can set aside half-hour blocks to take practice tests, and even take a weekend day to take a full exam. After self scoring these tests, you can work on the areas that gave you the most trouble. Even if you plan to explore more intensive options, this is a good way to get a head start. For some tests you can take that you won't have to self score, check out the free practice tests available here at Peterson's for the SAT, ACT, and PSAT.
The classroom test preparation option
The next most intensive option is a test prep class, whether an online course or an in-person course taught by a large company or local tutor or group. Online, you'll be guided through a process that helps with specific skills and can be tailored to your needs. Nevertheless, you'll need to be assertive to take advantage of this route. After all, it's up to you to set aside time and log in.
In person, you'll find more structure, but you'll have to get yourself there at a specific time and place, often on a weekend morning or afternoon, or after a long day of school. If taught well, a good class can help you with test-taking strategies, provide you with advice based on proctored practice exams, and offer a group environment in which other students might raise questions you didn't even know you had. If taught at too high, low, fast, or slow a level, or by an inexperienced instructor, a prep class can be at best a waste of time. Ask friends and teachers for recommendations and consider trying it out on a short-term basis to make sure it's a match.
The tutoring test prep option
Finally, you might decide to work one-on-one with a tutor for one or all of the sections of your test, whether it's the SAT, ACT, PSAT, or others. Such an approach is usually the most costly and personalized. You might, however, accomplish more in a shorter amount of time. A good tutor can administer diagnostic practice tests and quickly establish where you need to spend your time and effort.
As with a class, a good match between you and your tutor or coach is essential. If it seems like your coach is going to be too tough on you, asking you to do a lot of homework and pushing you to take your performance to the next level, we would encourage you not to throw in the towel. If you want to bring up your scores (that's the point, isn't it?) this might be just what you need.
Effort and timing in test preparation
With any of the three approaches above, you'll get out of it what you put in. As the saying goes, no pain, no gain. Your score won't go anywhere just because you paid a lot of money but expended no effort, and you won't learn anything or get any credit just for showing up.
One approach we suggest is to start out with some test prep materials and perhaps a short-term class in August (or January) of junior year. Depending on your experience and performance on the test in September or March/April, you might then begin some work with a tutor for the next test date. Or, you might consider a second or third pass at the test(s) in the fall of senior year, with more intensive work during the summer and early fall.
Don't give up — persistence will pay off. Taking your test six times doesn't make sense, but two or three times is fine. Most colleges will look at your higher scores, even on different sections of the SAT taken on different dates. Every year we work with seniors who bring up one or more sections on a December or even January test administration.
Remember: it's best to start early, but it's never too late to improve.