By reflecting on the Law of Kamma, that everything has a result. As we reflect on the Law of Kamma, we realize the results of being untruthful. We will be the ones who are responsible for being untruthful and the results of being untruthful will come back to us. By understanding this Law of Kamma, that we are responsible for our peace and happiness or our suffering, we realize that if were untruthful we harm ourselves. So understanding the Law of Kamma is very helpful for developing more Truthfulness.
As regards Truthfulness to ourselves, by building the power of our Compassion for ourselves, this allows us to open to Dukkha more easily. Building the power of unselfish emotions helps us to not create stories about the world but open to what it is. Unless we are truthful to ourselves we can't arrive at Truth. Also reflecting on how trust is so difficult to regain once it's lost. This can help us be more truthful so that people trust us.
Also understanding that we have to link our Truthfulness to Lovingkindness. Sometimes people are untruthful because they don't want to say the truth to someone because their afraid they're going to hurt the other person's feelings. So learning how to skillfully avoid saying things can help our Truthfulness to grow so we don't lie to people just so that we don't get blamed. We have more courage. Building the power of our courage also helps Truthfulness to grow.
In order for Truthfulness to grow we also have to understand what is true. So reflecting on the Four Noble Truths is also very helpful for developing Truthfulness.