One thing that helps me when I'm rolling my tongue is to have enough air behind it. It seems to take more air than some sounds. Let the tip of your tongue come very close to the bump behind your teeth (it's called the alveolar ridge) and then blow air over the tongue. The moving air will cause your tongue to flap if it's relaxed enough. Also, remember not to let air go out of the sides of your mouth.
You should be aware that Spanish doesn't use the rolled "r" in all places. For example, the previous answer about "perro" and "pero" demonstrates that sometimes "r" isn't rolled in Spanish ("perro" is rolled, "pero" isn't). The sound in "pero" is similar to the sound that English speakers say in the middle of the words "water", "ladder" or "butter": the tongue touches the alveolar ridge once. However, you always roll an "r" at the beginning of a word, and you can also roll it at the end of a word or in other places for emphasis. For example, if you are talking about "amor" and you really want to make it sound romantic, you can roll the final "r" to make the word longer.