There's many techniques to help in the letting go of self-images. To prevent the forming of self-images, we're going to have to build the power of our wisdom in the mind, our quality of awareness and insight. But usually we have to work from the other end, because our awareness and wisdom is usually not strong enough to prevent it yet. So whenever we start to see ourselves creating and selecting ideas, and then forming a self-image, it's very helpful to know how to balance this self-image so that we develop a certain amount of doubt, beneficial doubt in the reality of the self-image.
For example, the negative self-image comes usually because some people have a tendency to select negative memories about themselves, selecting all these memories, forming into an idea of themselves, an unchanging idea of themselves, and then believing totally in their creation. In order them to see that this idea is not true, they have to actually balance it with reflection on some of their beneficial qualities, their good past Kamma. In this way, they start to show the mind, that the image we're creating of ourselves has actually got another side, so how can it be totally true? A beneficial doubt starts to arise, and a certain lessening of attachment to this idea that they have created.
On the other side, there can also be some people who tend to form an inflated idea of themselves, and this leads to conceit. In order to balance conceit, it's helpful to reflect on the Buddha, Dhamma, Sangha, understand they're not enlightened, reflect on death, and ask themselves, "Am I free of greed? Am I free of hatred? Am I free of ignorance?"
In these ways, they can become a little bit more humble. If they strengthen their wisdom, then they may not believe so much in the memories that arise, and they can develop more equanimity towards them. The prevention of self-images really depends on strengthening our ability to see into the Eight worldly conditions; praise & blame, fame & obscurity, pleasure & pain, and gain & loss; strengthening our ability to be equanimous to memories that arise, and just see them as a memory. We need to also strengthen the quality of Forgiveness in the mind, so that we start to see the changing nature of who we think we are, because self-images come from a static idea of ourselves, a belief that we're unchanging.