The signals you want to produce will depend upon your motivation. If you want to make people agree with you, or at least make you feel like you agree with them, try to mirror their body language and posture.
Mirroring the body language of the other person subtly implies that you agree with them and it also promotes a sense of equality. If someone is standing whilst you are sitting, then this, depending upon the context can be interpreted as dominance or submission. Likewise signals such as having your arms crossed and body hunched compared to having your body open and relaxed can also give away disagreement.
When mirroring the body language of the other person, try not to be too obvious. If you notice a discrepancy between your body language, transition your posture and signals to mirror the other person slowly. If the other person is aware that you are imitating them, or feels that you are being awkward or stiff with your body language and posture, the effect will be ruined.
You can also display agreement by nodding. When we do actually agree with other people, we will often unconsciously nod our heads slightly to display this feeling. Customer service assistants and people who regularly deal with the public will also catch on to this habit and nod their heads automatically whilst talking to people whilst they work. Again, it’s important to be subtle. Don’t nod your head as if you had just been asked a question, but a smaller, gentler and continuous nod that isn’t obvious to the other person.
Smiling is also an effective way to display good will and get people to agree with you. The secret to smiling, however, is that you shouldn’t do it constantly. A constant smile is a false smile, and people don’t like false smiles. Instead listen carefully to what the other person is saying and smile when you think it is appropriate. You can nod your head in-between to keep up the good impression.
It’s also worth mentioning that psychologists have revealed that you do indeed ‘smile with your eyes’. There are muscles around the eye that are tensed and relaxed in a genuine smile that most people cannot control. To get a better sense of this distinction, try comparing photographs of yourself and your friends and family when they are smiling during a good memory compared to the forced smile you might see in a poorly acted commercial of magazine advert.
When closely examined the latter is almost uncomfortable to look at and observant people might be able to pick up on these smiles during conversation.