Kaplan has two solid books. I'd start with Kaplan 800, and then move on to MGMAT Number Properties to build your skills. If you're just starting out, focus on strategy tips as well. Here's what I suggest:
Choose a variety of values for Data Sufficiency. It may sound obvious, but don't make assumptions about unknown quantities on the GRE. "Numbers" can be positive integers, negative integers, decimals/fractions, or 0.
Slow down on Word Problems. Make sure you really understand the concepts underlying the question. One or two words can radically change the question. Don't rush these challenging questions, even if the math seems fairly obvious!
Watch out for extra steps. You may need to find 1/y, instead of y. Or you may be asked about the "ratio of girls in a class to boys in a class," but have to solve for the two parts of the ratio first. If you don't write down what the question is asking you to find, you will not see the end goal as clearly.
Pull out the important info first. Write down any key numbers, variables, or phrases from the question and write them down on your scratch pad. This is the step most students skip. Don't just scan the screen and start solving. Forcing yourself to slow down and process each piece of information will give your brain time to sort through it. This may lead you to find a faster way to solve!
Backsolve when there are numbers in the answer choices. Sometimes just doing the algebra will be the simplest way to the get the correct answer, but backsolving is a great strategy to check your work as you go. Go through the answer choices and plug each one into the question.
For Percent questions with unknown starting values, choose 100. This will make the Math much easier and you won't have to convert back and forth from the actual number to percents.
Pick Numbers as much as possible. Substituting abstracts like "x" for easy-to-worth-with integers like "2" and "3." Keep the numbers small and make sure they are allowed by the definitions in the question.
Be measured in your approach. Evaluate the question from ALL angles before deciding whether it seems easier for you to do the math traditionally or use a specific strategy. When you are practicing Problem Solving, try solving the same question in more than one way to see which was faster and more effective.
For Data Sufficiency, write down 12TEN on your scratch pad. It's easier to remember what the answer choices stand for (T = together, E = either, N = neither) if you use this acronym rather than the standard ABCDE.
Remember that "Y/N" and "Value" Data Sufficiency's are different. For "value" questions, a statement must provide a single numerical solution to be sufficient. For "yes/no" questions, either a firm "yes" or a firm "no" is acceptable but a single statement cannot be answered both ways. For "yes/no" questions, it doesn't matter how the question is answered, just that it can only be answered in one way.
Good luck!
