When you're at a job interview, your goal is to convince the interviewer that you can do the job. Part of the way you do this is by backing up the credentials listed on your resume with strong answers to the interview questions. Part of the way you do this is by simply appearing confident that you can do the job – nonverbal communication is an important contributor to the impression you make.
Dress the Part
It used to be necessary to wear a suit and tie for every job interview. In tech today, that's no longer the case. Try to find out what's appropriate for the company before your interview. Wearing the wrong clothes will undermine your confidence; wearing clothes that make you look like you fit in will help the interviewer picture you doing the job. Whatever style of dress is appropriate, make sure you wear something you like and feel comfortable wearing.
First impressions form almost immediately and carry a lot of weight. Make eye contact, shake hands firmly, and don't be hesitant when you walk into the room. Sit up firmly in your chair – a chair with a firm back where you'll sit up straight will help you present better than a comfy chair where you slouch down. Keeping your feet solidly on the floor will help you maintain good posture. You don't want to be rigid, but don't be fidgety, either.
If you don't seem interested in the position, the employer probably won't be interested in you. Lean forward during the conversation, but be careful not to intrude on the interviewer's personal space. Avoiding eye contact makes you seem hesitant, but don't engage in a staring contest. Be aware of your voice: tone and speed can make you seem either bored or engaged.
Try not to cross your arms; it's a defensive gesture. It's better to keep your arms loose and to talk with your hands, as long as you don't wave them around crazily. It's also fine to smile; if you look like you're enjoying the topic, the interviewer will enjoy talking with you.
When you do practice interviews, practice your body language as well as your responses to interview questions. It may be harder to overcome habitual behaviors than to come up with answers for tricky questions, but presenting yourself well is an important part of succeeding at interviews.