Most students take the SATs twice: once in the spring of 11th grade and once in the late fall of 12th grade. Many students will also take SAT Subject Tests, which are given on the same day as the regular SAT: you need to choose one or the other, so you really do need to plan carefully. And then there are the AP tests and Regents exams. It's best to spread them all out as much as possible.

I have found that most 11th graders are not ready to take the SATs seriously, no matter how much you nag -- I mean -- encourage them.
12th graders, by contrast, are suddenly VERY aware that their ENTIRE future is riding on the test, and are often heard demanding of their parents,"why didn't you TELL me?!!"  (Aren't teens fun?)

That said, the summer before 11th grade it would be good for them to participate in a low-pressure academic program which will build their reading, writing, vocabulary and math skills. Unless your child is self-motivated, don't waste your money on an SAT prep program.

The summer before 12th grade (presumably after they've taken the SAT once) is a good time to take an SAT prep class. By then, they will be motivated to study, especially if they realize that their grades the first time around are not quite high enough to get them into their dream college.

The Prep programs offered at many schools are a good price (as they are somewhat subsidized), but they are cookie-cutter franchises that provide lots of drills and not much individual instruction.

Private tutors can cost $150/hour and up, so shop around!

I highly recommend registering for the SAT Question of the Day from the College Board. It's just that: one question a day. It's an excellent way to get very familiar with the test .