Place yourself in an informal setting where you can talk with friends. Like some other languages, Japanese has different words which are used depending on the familiarity between the two people speaking. Practice the words and phrases you would use to refer to those ones you know or are close to. Interchange this with terms a speaker would use to talk to a stranger. You may practice by going to a restaurant with friends who are fluent in Japanese. When talking to a stranger to ask directions to get to the restaurant, for instance, use the word forms appropriate to communicate with someone you don't know.
Attend a formal dinner for a grandparent and use Keigo, the honorific form of Japanese. This form is used for those of an elevated social standing like elders, customers and teachers. This form of the language involves a speaker referring to himself in a humble manner and talking to the other person in an elevated manner. Do not be discouraged if this is difficult. Even native speakers need to practice.
Attend a major event like a sports game or concert in Japan with friends. In this way you would be able to speak with a large range of people, some you know, many you do not. Some may have different Japanese dialects like Hokkaido-ben or Ibaraki. In this manner, you can be in a relaxed setting but still be able to use your language skills.