Purchase a book or pamphlet of sample GED test math problems. These are available online or at your local bookstore.
Practice math problems with a tutor or friend who knows how to answer the problems correctly. After all, if you practice the wrong concept, you will be setting yourself further back than if you never practiced at all.
Write some problems on 3-by-5-in. index cards. This makes the problems more accessible than if you were practicing them directly from the book.
Carry a set of ten index cards with you everywhere you go. Whenever you have a spare moment -- such as coffee break at work or between breakfast and when you leave the house for work -- do one or two problems. By spreading the problems throughout the day, practice won't seem like a chore, and the quality and quantity of your study will be better than if you just sit for three or four solid hours doing problems.
Check your answers at the end of the day. Algebra problems can be checked by putting the answer into the original problem, but other problems will need to be looked up in your book. By doing this, you can focus on your weak areas and use your strong areas to boost your confidence.