Gather all your notes from your high school mathematics classes together and go through them to re-familiarize yourself with the concepts. Using materials that you have already seen (and have written yourself) will make it much easier to draw on what you already know. Be sure that you take note of any concepts that you highlighted.
Look over your past tests and go through each one as though it were a study guide. Take care to pay special attention to the problems that you got wrong and figure out exactly what you did not understand.
Create a list of important algebraic formulas that you have difficulty remembering, and then commit them to memory. Having formulas at the forefront of your mind will save you some valuable time during the test. Here are some useful ones:
a² - b² = (a - b)(a + b)
(a + b)² = a² + 2ab + b²
(a - b)² = a² - 2ab + b²
Visit your local library or bookstore and borrow or purchase study materials geared specifically towards studying for algebra exams. Amazon.com offers a few promising and affordable study guides for college algebra, including "College Algebra Demystified" and "College Algebra Solved!" By this time, you should be well aware of the concepts that give you the most trouble. Use these books to really brush up on those skills.
Practice what you've studied by taking tests out of a purchased or borrowed book, or you could always check out some of your online resources. The University of Missouri-Columbia offers several versions of college algebra tests at: http://mathonline.missouri.edu/. Remember that carefully looking over the test afterwards is as important as taking it.