It is amazing the difference in effort you will get from worksheet to worksheet. Granted the amount of effort may vary immensely from year to year depending on the group of students you have.
For many middle-school children, math is a real challenge. It is at this age that the concepts begin to get more abstract, and work goes beyond "concrete" ideas like adding and subtracting. It's no surprise that at this age, math can become something the student avoids, or begins to dislike.
Therefore creativity is a must for worksheets to be successful : Regardless if you are trying to review math, science, reading, writing, health, or social studies, your goal should always be to try and create something that will generate desire in the students to actually want to do it. If you can do this, the battle is practically over already.
You can find worksheets for a wide range of courses--almost any course you want to teach your children. These include spelling, writing, English, history, math, music, geography, and others.