A few tips for Data Sufficiency:
1) Make sure you are comfortable with the answer choices and don't really have to think about them when solving problems. For instance, students will often ask "We got B, but couldn't it be C?" This indicates a misunderstanding of the answer choices, as we'd only get to C if we ruled out both statements as insufficient.
2) Try to rephrase the question. Sometimes the question is already pretty simple ("What is x?"), but if it's more complicated, you can greatly increase your accuracy by simplifying or reworking the question to reflect any initial information you have. Sometimes one of the statements will directly answer your rephrased question.
3) Don't confuse the question with the statements. Always keep in mind that the question is just that: a question. If it asks "Is x a prime number greater than 10?" don't assume that x is prime. With that in mind, if you rephrase the question, keep the question mark at the end. Many people miss DS problems because they unconsciously slip on this, thinking something like "Ah, if the question is true, statement 1 could be true. These are consistent." That's not the task!
4) Have a consistent process. Try to prove each statement insufficient, and don't move on until you've considered the cases in which you might get a different answer to the question. If the statement is insufficient, take a moment to consider what it *does* tell you, so you'll be better prepared when it comes time to consider combining.