Although using a condom or other barrier isn't a guarantee that you won't get any STD, condoms are your best way to avoid getting infected while still having sex. Even the STDs, such as HPV, that condoms don't prevent entirely, have reduced transmission rates when condoms are used. Other than abstinence, consistent condom use, which means using a condom every time you have sex with every partner you have, is the best way to prevent STDs. Make condoms enough of a habit, and one day it will become inconceivable that people have sex without them.
2. Multiple Partners
It's pretty straightforward math -- the more partners you have, the more likely it is that you will be exposed to an STD. Furthermore, people with multiple partners tend to choose partners with multiple partners, so each individual you are having sex with is probably more likely to have an infection than someone with whom you would choose to be monogamous.
Young people are far more likely to be infected with STDs than older people. This is for several reasons. First, young women, in particular, are more biologically susceptible to STDs than older women. Their bodies are smaller, and they may be more likely to experience tearing during intercourse. Their cervixes also aren't fully developed, and are more susceptible to infection by chlamydia, gonorrhea, and other STDs. Young people may also be more likely to engage in sexual risk taking - particularly under the influence of alcohol - and more likely to have multiple partners. However, on the upside, recent research does suggest that they're also more likely to use condoms.
4. Alcohol Use
5. Illicit Drug Use
Illicit drug use can make decision making difficult. People who have sex under the influence are more likely to engage in risky sexual behaviors: e.g. sex without a condom or other form of protection. Illicit drug use may also make it easier for someone to pressure you into engaging in sexual behaviors where you wouldn't otherwise choose to participate. Furthermore, injection drug use, in particular, is associated with increased risk of blood-borne diseases - such as HIV and Hepatitis - that can also be transmitted during sex. So injecting drugs, or having sex with someone who injects drugs, is a double whammy of risk for these serious illnesses - particularly if they do not have access to clean needles.
People who trade sex for money or drugs may not be sufficiently empowered to negotiate that that sex be safer sex. Furthermore, partners acquired in this manner are far more likely to be infected with STDs than people in the general population.
Note: Some sex workers, particularly those who have made an informed choice to enter their professions, are highly concientious about safe sex and prevention. Risk varies according to individual behaviors, just as it does for people who don't engage in commercial sex.
7. Living In a Community With a High Prevalence of STDs
Some people only date one person at a time, but still date a large number of people each year. This is referred to as serial monogamy. The danger for people who practice serial monogamy is that each time they are involved in an "exclusive" sexual relationship they are likely to be tempted to stop using safer sex precautions. But monogamy is only an effective way to prevent STDs in long term relationships where both of you have been tested. Since some tests aren't reliable until you've been infected for some time, many serially monogamous relationships don't last long enough for that to even be a viable option.
For many people, the biggest worry about having sex isn't STDs... it's pregnancy. Many heterosexual couples choose birth control pills as their primary form of contraception. However, once protected from pregnancy, some people are reluctant to use condoms as part of their sexual routine. This can be because they are afraid of implying their partner has a disease or because some people don't like using condoms, and birth control pills are a good excuse. Dual protection, using both birth control pills and condoms, is the best option - particularly since pills can reduce your risk of PID, but it is an option not often taken.