When you ask another person for sex, you are asking them to take a risk. If they say no, it doesn't necessarily indicate that they don't like you. There are a multitude of reasons a person may decide against becoming intimately involved. If you accept their decision respectfully and with kindness, you stand a much better chance of getting lucky with them at a later date. After all, you've planted the seed and shown that you can be mature, friendly and graceful in the face of adversity, which many people find very sexy.
When they say yes, it's a great idea to discuss what each of you has in mind. This part takes practice, too. Since there are numerous ways to have sex, being on the same page before you start getting hot and heavy can help to prevent the over-stepping of personal boundaries. Communication is the most important key to becoming the amazing lover you were born to be. And to become a successful communicator takes practice, practice, practice.