It is unlikely that a person can deliver a brilliant sermon without practice, but practice does not guarantee a brilliant sermon. Practice does not make perfect but practice does make possible.
In preaching, sports, music entertainment, parenting or any other field we practice, we think about the possibility that practice can offer. We might think about the glory we may gain if we practice and 'nail it' when it comes time to deliver. We think of the personal sanctification we might get if we get the results we worked hard to obtain. However, we may forget that our practice may lead to another's possibility.
Each week, choirs around the church world practice singing their praise songs and hymns. When it comes time to offer those songs in worship, the practice of the choir offers a possibility for the choir to have a level of fulfillment to be sure. However their practice also offers a possibility to those in the congregation: the possibility of being reconnected to the transcendence of God and that which is beyond.
Your spiritual practices also make it possible that you may encounter the divine, but more often than not, your spiritual practice creates the possibility of others to encounter the divine through you.
The possibilities of practice do not end with you.