When getting home after an interview, first thing you should always do is write down every relevent information, positive and negative. Take note of everything that happened during the interview. Write down the name of the people you met, the questions and topics of conversation you had with each of them. You may feel like you can remember all these details by yourself, but after a few interviews, everything gets a bit confused.
Send a personalized email to every person you met during the interview, to thank them for their time. Make them unique, so they will seem more sincere than a grouped email. It also shows your employer how motivated you are for the job.
How well did you do? Did you manage to answer all the questions? If you feel like you could have dealt with a few questions a bit better, take a few minuts and think about what you would say if you were asked again. It's probably the last thing you feel like doing at the moment, but it's the best way to improve your interview skills (for next time).
After doing all that, it's time to move on: don't spend you days overthinking what could have been done, or waiting for the results. Live you usual life, and add a few fun activities to take your mind off things (a nice restaurant with some friends, a movie, a museum... just get out there). During the interview, ask how long the decision-making will take: this way you won't refresh your mailbox every two minuts for the days to come.
If you haven't heard from the employer after the day they were supposed to let you know of their decision, send another email. If you haven't been given a specific date, wait around 10 days before contacting them. Don't just copy/paste your previous thank you email ! Thank them again, and let them know that you are very interested by the position and that you are still waiting on their decision.
Don't put all your eggs in the same basket! If you apply to other positions, it will be a lot easier to accept a refusal than if you only applied to the one position. We obviously wish you to succeed, but the job market is quite tense, and one must be prepared!
Best of luck!