They should not be under conscious control. Consciously selected miracles can be misguided. Miracles are natural. When they do not occur something has gone wrong. Miracles are everyone's right, but purification is necessary first.
Miracles are a kind of exchange.
Like all expressions of love, which are always miraculous in the true sense, the exchange reverses the physical laws. They bring more love both to the giver AND the receiver. The use of miracles as spectacles to induce belief is a mis- understanding of their purpose. Prayer is the medium of miracles. It is a means of communication of the created with the Creator.