The best approach is, again, to act naturally. Virtually everyone gestures in normal, relaxed conversation, although the amount and kind of gesturing varies depending on cultural background. But everyone gestures, and the best way to prepare for acting naturally is to get a sense of your natural gesturing patterns and those of others. Begin to observe yourself and your friends in conversation and note how and how much you and they gesture. In interviews you should try to duplicate your natural patterns, unless these patterns are distracting.
Appropriate gestures are meaningful. They emphasize, qualify, or illustrate the words they accompany. For example, if you say you have three major reasons for wanting to work at ABC, Inc., you might count off your three points using your hands and fingers. If you are saying that it took a long time to accomplish a goal, you might emphasize and illustrate "long" by holding your hands in front of your torso some distance apart. If you gesture continuously, your gestures will lose their emphatic power.
On the other hand, gestures may become too emphatic, appearing to be intrusive and aggressive. A wagging finger or a handshake that crushes can create a bad impression.
Most interviews open and close with handshakes, so your handshake contributes significantly to the overall impression you make. In addition, humans respond to physical contact touching very strongly. A good handshake is neither limp nor overbearing. It is definite, not reluctant, and relatively brief It may seem silly to practice your handshake, but I strongly recommend that you try out your hand on friends and ask for feedback before you start interviewing. If you are foreign born or grew up in a cultural ghetto, practice with natives like all gestures, handshakes vary significantly from culture to culture.
To gesture freely and to look alert without seeming anxious, you need to maintain a good seating posture. You should neither sit on the edge of your chair nor lounge in it. When you first seat yourself, aim to get your buttocks firmly at the back of your chair, which will leave your torque to move and your arms free to gesture while giving support to your back.
And watch those legs. Legs crossed at the knees or ankles are fine. Hooking your ankle over your knee is not so hot men beware. Women are rarely tempted to sit this way when attired in a dress or skirt, but many men adopt this posture immediately and may unwittingly give offense when doing so. Your leg is a formidable barrier and you don't want to put it between you and your interviewer. Also, you want to avoid the temptation to lean on your leg and play with your pants. Picking nits or playing with socks are not meaningful gestures, and if you busy your hands this way, they won't be available for more appropriate gestures.
Do not be afraid to move during the interview, as long as you do not fidget or move intrusively in someone else's space. When you move, move with confidence. Enter the room shoulders squared, head up, and treading firmly. Do not shuffle or appear to apologize with your body for taking up space. Some people have a curiously strong inclination to play with items on interviewers' desks or shelves. Stifle this inclination if you are prone to it. I've heard a surprisingly large number of interviewers complain of such behavior, which they experience as roughly equivalent to trespassing.
When in doubt about appropriate nonverbal communications, try to mirror your interviewer. When feeling in rapport, humans naturally mirror each other's posture, gestures, and movements. Observing two people in friendly conversation, it is not unusual to see both with their legs crossed the same way and their hands in roughly the same attitude. When people feel defensive or uncomfortable with one another, however, mirroring generally stops. One person may sit with hand covering mouth while the other gestures frantically; one may sit in a lounging position with both feet on the floor and legs spread widely while the other sits rigidly with legs tightly crossed. If you pick up such discrepancies, you may want to adjust your behavior.
Your voice is also part of your nonverbal self. Your words, of course, convey meaning, but so does your tone of voice, pace of speech, amount of inflection, use of vocalized pausing, and volume. Vocalized pauses are the ums, ahs, you known's, OK's, and other meaningless sounds or phrases that may fill the gaps between meaningful utterances. You should avoid peppering your speech with them, but you may use them without being aware of doing so. Eliminating them can, therefore, be difficult. If you try to become aware of and eliminate vocalized pauses only when you go for interviews, chances are good that you will only become more self conscious, which is likely to increase your nervousness and your inclination to vocalize pauses. Try, instead, to become more conscious of your speech patterns outside and before interviews. Think of yourself as replacing vocalized pauses with brief silences and get accustomed to tolerating silences, both in others and in yourself.
Practice sessions with videotape are an invaluable tool in preparing for the nonverbal as well as the verbal aspects of interviewing. Videotaping allows you to see and hear yourself much as others do and enables you to fine tune your performance. Ideally, you should practice with an experienced coach. Sometimes, however, they are hard to find or very expensive because they work primarily with high level executives being trained for TV appearances and the like. Communications counselors often provide coaching; as noted you can often find good coaches by calling a university affiliated business school in your area and asking for a referral from someone in the management communications department. Before you engage a coach, however, make sure that he or she uses videotaping as a tool.