Sources
I used just tons of books and other sources to help me study. Here's my take on them:
(Disclaimer thingie: I don't make money from this)
Clinical Laboratory Science Review, a bottom line approach (4th ed.), by Patsy Jarreau: This thing is your best friend! It reviews everything, and it includes great memory tricks for studying. It basically boils everything down to what you need to know. Yes, it leaves some things out, but if you had nothing else to go on and just knew everything in this book, you would pass your boards. Each section also includes some review questions with answers and explanations. You should be able to get it for about $45 (worth every penny). The ISBN is 9780967043425. Sometimes Amazon has it listed higher, but you can always get it from Rittenhouse. I also ended up going to Office Max to turn it into a spiral-binding for $4. It made studying much easier since I used this book so much.
Clinical Laboratory Science Review (3rd ed.), by Robert Harr: This is really good for practicing for your boards. The book itself is 500 pages of questions, with answers and explanations on the same page, and a practice test in the end. Also, it includes a CD with a little program that lets you practice in the areas you want. You just pick an area, a difficulty level, and how many questions you want, and you can take a mini customized quiz. The CD also has full practice tests too. If you can get your paws on this book to study for the boards for about a month before you take them, it's worth it. I saw very similar questions on my real test. The ISBN is 9780803613737.
Quick Review Cards for Clinical Laboratory Science Examinations by Valerie Dietz Polansky: This is not a book, but a set of review cards. Note: these are NOT FLASHCARDS. It's a big stack of just about all the MLS info organized onto big cards. Personally, I loved them. They have all kinds of information put into charts for you. You can very quickly and easily look up items. I'm sure I'll post a few on this site as they're so nicely set up. These cards cover all the different sections, including things like "General lab practice" and "Management and education." They cost me a little over $40 + a box to put them in. If you're in a program with several other students, you could pool your money and buy 1 set, and pass the different sections around to study. ISBN is 9780803604599. I also just saw there's an e-version for a Kindle. I can't tell if that would be better or worse than a physical copy. 
BOC Study Guide (5th ed.) by ASCP: This is a book from the ASCP (you know...the people who write your certification exam...) that's similar to the Harr book. It's all questions. Again, very good questions. It's broken into the different sections and has all the answers at the end of each chapter with little explanations. Some questions also have a tag of "MLS Only," so if you're a technician not a technologist you can tell them apart. I think it'd be very helpful to have access to either this book or the Harr one to do practice questions. I believe this book is more expensive than Harr, but if you somehow have free access to it, you wouldn't have to buy the Harr book. ISBN is 9780891895879. 
Medical Parasitology, A Self-Instructional Text (5th ed.) by Ruth Leventhal: This is only about parasitology, which is actually a very minor part of the certification exam. Do not buy this just for studying for your exam; you don't need it. But, if you're like me and find it a neat topic, this is a great paperback text to get you started. You can find the 5th edition pretty cheap online ($10ish). I know there's a newer edition, but it only has very minor changes. I enjoyed this text, and you really can use it to teach yourself about parasitology. ISBN is 9780803607880. - CDC Website: On a related parasitology note, the CDC website is an excellent resource. They have lots of details, pictures of lifecycles, etc. Check it out.