Parents have often found that role playing with teens to react responsibly to difficult situations increases the likelihood that the teens will make good choices. Ideally, you've started role playing with your teen during middle school, as some of these risky scenarios can arise even then. If not, later is better than never -- so get started.
First, make sure the scenarios you role play are specific to a particular risk. Common situations to role play are being offered drugs or alcohol, being pressured sexually, or getting a ride with someone who is now drunk. Role playing just one of these isn't enough because the responses your teen can give and the type of follow-on pressure for each scenario is different. So you need to practice them all. In that spirit, the role play shouldn't just be a specific situation; rather, dig deep with your teen into each scenario. After your teen's initial "no," what are the likely comebacks he/she will hear. How will he/she have to stand by his/her "no?" Even if you've role played all these scenarios with your teen, it might be useful to go over them again before events where he/she is likely to encounter one, such as prior to going to a party. As a last resort, you and your teens can identify a code word they can use on the phone with you, so you'll know to come get them even if they don't want to ask in front of their peers. Your teens might not admit it when you set up the code word, but they will be grateful they always have a backup plan if necessary.
While you might feel awkward about role playing (which is natural), it's still the most effective way to help your teen deal with the challenges of being teen. As you role play, you're teaching your teen how to stand-up for him/herself, how to exercise his/her own judgment, and increasing his/her self-confidence.