First impressions really are all-important, so make sure that you give yourself enough time before the job interview to go to the bathroom and freshen yourself up.
Use a relatively firm handshake at the start and end, and maintain eye contact wherever possible. Sit up straight, look interested and focused and try to come across as being confident - if you look like you can do the job, then you're a lot more likely to get it.
Sometimes it can come down to the small details, so if it's between you and someone who fidgeted for the full 20 minutes, then you might just have the edge...
When trying to make a positive first impression, it stands to reason that you should try to project a positive personality, so stress the benefits that you can bring to this role and why you think you could be a success in it.
If you have any grievances about your old job or employer, then now's definitely not the time to air them. It's great to show personality (as we'll explain in a second), but not when it shows your negative side. Employers aren't going to want to hear about how bad your former boss was - it will just make them imagine what you could be saying about their company in future years.
So if you've got negative reasons for leaving your old job, then make them sound constructive or, at least, try to remain upbeat and professional.
As mentioned above, it's important to show your personality and attempt to strike up some kind of rapport with your interviewer. Companies will often speak to several candidates back-to-back, so any positive impression that you make could give you an advantage.
This doesn't mean that you should take on a different persona though. Try to relax and feel free to laugh, but judge the tone of the interview before you make it too light-hearted.
If you're lucky enough to discover that you both grew up in the same area or share the same interests, then make the most of it. Interviewers love to see an indication that they could actually work with candidates if they give them the job.
Take your time
If you're unsure about how to answer anything then it's ok to take your time, have a sip of water or even ask for a few seconds to consider your response. It's far better to give a strong answer after a slightly awkward silence than sprout gibberish because you were caught off guard. Equally, if you're unsure of the meaning of any questions, then ask for clarification.
End it politely
While it may not be as crucial as the first impression, it's still important to end the job interview in the right manner. Shake their hand, thank them for their time and let them know that you look forward to hearing from them soon.