Students are forgetting information, because they don't feel that it's useful. Usually, the things you remember, you found important when you learned it. So how can you help students to remember topics that were taught years ago?
I think some great ways to do this, is for teachers in later grades to assign review questions on every homework assignment they create. It doesn't have to be much, just a few questions to jog the students memory. That way, students will at least have an idea on what to do if a 7th grade math question pops up on the SAT.
But it's not just about remembering. There has to be a reason why students aren't remembering topics. I think a big part of this is the fact that students simply don't find this information useful. When am I going to need the formula for the area of a triangle, or the turning points of World War 1 when I'm an adult? Having students review topics from many years ago will help, but if you can get the topic to stick into their minds in the first place, you wouldn't need as much review.
In Ellin Oliver Keene's chapter in "Adolescent Literacy" edited by Linda Rief, Kylene Beers, and Robert E. Probst, she talked about what it means to truly understand something, and it goes along with what I'm saying.
Students constantly take notes and answer questions on worksheets, when they should be asking their own questions on the topics, and using real world problem solving methods to answer them. Another way to make topics seem meaningful is relating topics to a students preference, and individualizing a lesson best suited to a student.
The key is getting a student to like a topic, so that they will see it as "meaningful" information. By solving this, not only will students be learning topics how they want in a way they like, but they will be taught how to solve real world issues. After all, isn't school supposed to prepare students for the real world?
Beers, G. Kylene, Robert E. Probst, and Linda Rief. Adolescent Literacy: Turning Promise into Practice. Portsmouth, NH: Heinemann, 2007. Print.