Sometimes we intend to do good but cause destruction instead. Do not bombard too many regulations on a non-Muslim, because he might not be able to handle it. It is the same with a Muslim whose iman is weak. The priority is to build up that person's faith, instead of flooding him with injunctions that he is not at a stage to fulfil. Remember that many of the regulations are addressed to the believers specifically, and not to mankind in general. We have to utilise wisdom, because otherwise, the da'wah efforts will backfire and cause more damage than good.
Wait for the green light: a sign from Allah. You will find yourself talking in a gentle manner. When people are attracted to you, listen to you and seek your company, then you can impart the knowledge in small steps. You don't force-feed the baby with food he is not ready to digest. Similarly, we can't give an ill person a huge meal. It may be healthy for them theoretically, but if given at the wrong time it can kill them.
Constantly make du'a for them, but don't think that because you are doing that, you are superior to them in any way. Don't engage in da'wah with the intention that you are better than them because you don't know what is in store for you or the other person.
Da'wah is the work of Allah SWT. Allah gave quality to Rasulullah SAW as a mercy to mankind. Be merciful to what is on the earth and Allah will be merciful to you, because only the merciful will obtain the mercy of Allah.
At work, do not be in a state of hypocrisy and expect your co-workers to have respect for you or Islam. Behave with the intention that Allah is watching. Your steadfastness and sincerity will open the road for you.
These are basic common sense principles, but this is the genuine da'wah. Connect yourself to Allah and be humble to the gift that Allah gave. Remember that we are the slaves of Allah. We are merely doing what our Master wants us to do, not what we want to do and feel like doing.
Nuh AS preached for 950 years and barely anyone paid heed. Very few people followed the path of Isa AS during his time on earth. Rasulullah SAW preached for 13 years daily, and less than 100 people embraced Islam during this time. The main da'wah originated in Mecca, but Islam flourished in Madinah and the core supporters of Islam were in Madinah. Allah is teaching us that we have a job to do, but the result is His business, not ours. We can work in one place and Allah can spread your words to another location, which you never intended.
We do not look for the physical results of the victory of Allah, but we believe that Allah is watching as we carry our message. Otherwise, we will get disheartened and we will not continue. Only Allah has the power to achieve the opening for us, but only if we trust in Him. We are just farmers and we keep ploughing on. We have to do what we should do in the face of hostility and resistance, continue with determination, kindness and generosity.
May Allah put in our hearts the right responsibility and to work purely for Him in His way, with complete faith and tawakkul (surrender). Barakah and light are from Allah alone.