The NASAR certification exam, or a class? Two different animals. I was/am? a NASAR SAR TECH Evaluator (being a physician got in the way) and taught lots of people in a non-NASAR course to prepare people for the exam.
The book Fundamentals of Search and Rescue, offered by NASAR, is pretty good and will prep you for the test. The test is 100 questions, multiple guess. Nothing is taught in the class that isn't covered on the test, it's all important.
The area that people always have trouble with on the practical exams is the land nav - especially the distance estimating portion of it. I suggest that you get a long (100') measuring tape and measure out some long (200-300') distances on different types of terrain, and practice your pacing skills (wearing the clothes/shoes/pack you'll be using for the exam) until you consistently get it right (within 5%, the standard is 10%). Also practice taking compass bearings.
The rest of the skills are pretty easy: There is a required pack inspection - you MUST have everything that is on the list. As an evaluator I don't care if you ever carry that stuff again, and in fact I disagree with a couple of items, but for the exam you MUST have each and every item or its a fail. The tracking eval is pretty simple - 10 prints in a print box, show how to mark them and use a tracking stick to find the missing one. Area/Line search is tedious (both to set up, watch, and do), you need to find playing cards hidden in the brush. Look up, down, backwards, sideways.
Get a piece of clothesline (some reasonable diameter, not parachute cord) about 10' long and practice the knots, which are all variants of the figure-8.
Where will you be taking it?