Abstinence.

What Is It?
Sexual abstinence is the practice of refraining from some or all aspects of sexual activity. Common reasons for practicing sexual abstinence include:
* religious or philosophical reasons
* material reasons (to prevent conception - undesired pregnancy - or sexually transmitted disease transmission);
* psycho-sociological reasons (e.g., clinical depression, social anxiety disorder, increasing testosterone in males, or negative past experiences); or,
* legal injunctions requiring conformity. (e.g., age of consent)

How Does It Work?/How Do I Do It?
If two people don't have sex, then sperm can't fertilize an egg and there's no possibility of a pregnancy. Some forms of birth control depend on barriers that prevent the sperm from reaching the egg (such as condoms or diaphragms). Others interfere with the menstrual cycle (as birth control pills do). With abstinence, no barriers or pills are necessary because the person is not having sex.
*A couple can still have a relationship without having sex. If you've made a decision not to have sex, it's an important personal choice and the people who care about you should respect that.
*If it seems like everybody else is having sex, some people may feel they have to do it, too, just to be accepted. Don't let kidding or pressure from friends, a girlfriend, a boyfriend, or even the media push you into something that's not right for you. The truth is that most teens are not having sex.

Who Can Use It?
Anyone, you don't have to be a virgin to practice abstinence. Sometimes people who have been having sex decide not to continue having sex. Even if a person has been having sex, he or she can still choose abstinence to prevent pregnancy and sexually transmitted diseases (STDs).

How Well Does It Work?
Abstinence is 100% effective in preventing pregnancy. Although many birth control methods can have high rates of success if used properly, they can fail occasionally. Practicing abstinence ensures that a girl won't become pregnant because there's no opportunity for sperm to fertilize an egg.

Protection Against STD's?
Abstinence protects people against STDs. Some STDs spread through oral-genital sex, a**l sex, or even intimate skin-to-skin contact without actual penetration (genital warts and herpes can be spread this way). So only avoiding all types of intimate genital contact can prevent STDs. Avoiding all types of intimate genital contact — including a**l and oral sex — is complete abstinence.
*Only complete and consistent abstinence can totally prevent pregnancy and protect against STDs. Because a person does not have any type of intimate sexual contact when he or she practices complete abstinence, there is no risk of passing on a sexually transmitted infection.
*Consistent abstinence means that someone practices abstinence all the time. Having sex even once means that the person risks getting an infection.
*Abstinence does not prevent AIDS and hepatitis B infections that come from nonsexual activities like using contaminated needles for doing drugs, tattooing, or taking steroids.

Risks/Health Concerns?
Abstinence is 100% risk free with no health concerns if you are practicing complete abstinence.

Any costs?
Abstinence is free; unless you want to buy a purity ring to show your commitment to abstinence.

Info from; Kids Health, Wiki! and my brain.. some of it anyways. ;p