It's very important to follow up after a job interview, because even if you think the interview went badly, keeping in touch may improve the interviewer's perception of you. It reflects persistence, and it's also polite to thank the hiring manager after the interview. Remember to also send the note within two days of the interview; although, if you've passed the two-day mark, a late response is better than no response.
There is such a thing as being too pushy, so keep it light, cheery, and professional. Here is a sample of the kind of email you should send:
It was great meeting you today, and I appreciate you taking the time to interview me. I'm excited to be considered for the (name the position) as well as all of the opportunities the company presents. I had a good time discussing my passion of (insert what you're passionate about) and really enjoyed learning more about (insert what new tidbit you learned about the company).
Thank you for your consideration, and I look forward to following up with you.
Remember, you should have asked them when you'll hear about their decision at the end of the interview. If you've forgotten to do that, you can politely add the question to your follow-up email.