How we change what others think, feel, believe and do
Be reliable. Do what you say you are going to do. Turn up on time. Always repay debts.
If you make a promise then always do your very best to keep it. If you cannot, apologize work to make amends so they know it was not through lack of care.
Also be predictable and 'normal'. Do not do things that others do not expect.
Make a particular effort to show that you are reliable. Make a promise in order to keep it. Tell them about how you have been reliable in the past.
If you do not keep a promise or are unreliable in some way, apologize profusely and give a strong excuse as to why this happened.
I meet someone and say I have to do something I promised to someone else and it will take five minutes. In exactly five minutes I return.
We have a deep need to predict how others will behave in order to feel safe and so achieve a sense of control. When others behave in unpredictable ways, including being unreliable, then it causes confusion and we feel unable to trust them.
Note that being reliable is not something you do some times. It has to be all the time. It only takes one failure for them to consider you as unreliable. Apology can recover the situation, but this is a 'silver bullet' which you can only use very few times.
People who try to be nice to others by saying yes a lot and promising whatever is asked can easily be thought of as unreliable when they do not complete things as expected. It is often better to 'do less, better'.
When you do not have time to let people find you are reliable then you can accelerate the trust and rapport by giving them examples and stories by which they can conclude you are reliable without having to wait to gather lots of evidence.