Sincerity is indeed one of the most, in fact the most difficult quality to acquire.
The very first step in being sincere is to stop the automatic justifications that we give to all that we think and feel and do. Secondly, we have to see whether our thoughts, feelings and actions are taking us closer to the goal or farther away from it. The reference point of sincerity here is the Divine Goal before us. But to recognise it needs mental honesty so that we do not deceive ourselves or the Divine. We should be conscious and know the true motives behind our actions. A man may be very sincere with regard to his workplace or his family but he may be doing all this for personal gain, ambition or even duty etc. But these attitudes may be placing a barrier between him and the Divine.
Therefore the aspirant must reorient himself and do these things in the spirit of service to the Divine without any other expectation but the joy of service. Thus even while he is outwardly engaged in the same actions his inner being is growing more and more sincere. later he may go one step further and feel the inner impulsion of Truth towards certain activities but his old way of life, even though done with the right attitude may come in the way. He is now attached to a certain way of being, of living which he thinks to be the highest and the best. But the time for these things is over. He should be therefore able to leave at once without regret all that was a help in the past but is becoming a hindrance today.
Thus sincerity grows as we grow. It is also useful to remember that the light of our mind, even the highest mind may not be able to show us all the hidden corners where all kinds of motives lurk. A surer light is the psychic being and hence the first important thing should be to find and unite with the psychic being. then only the true sincerity can emerge and we can march forward without being fooled by these subterfuges. We can of course ask for sincerity from the Divine and pray that it grows.
Let the Grace and Love and Peace flow. The key here is to forget oneself and become as impersonal as possible. It is also very important to be in a state of constant humility and know that nothing belongs to us. All power and beauty and knowledge belongs to the Divine and if He has given a fraction of it and let it pass through us it is entirely His Grace. This humility is our safeguard, the rest doesn’t matter. Flowers remember the sun and reach out to it, while they are self-oblivious of their beauty and fragrance. We as human beings monitor ourselves, and that habit interferes with the free play of the Divine. To forget ourselves, including what we have or what we are and instead think only of the Divine is the right attitude to take.