The heading is intentionally provocative. Normally we want to encourage preaching but there is one part of a service in which preaching should be forbidden. That is every part of the service which is not actually the sermon itself. It is very tempting to preach when introducing songs or leading the prayers or giving the notices. I strongly recommend resisting that temptation.
First, it is simply annoying as a member of the congregation to find oneself poised to worship the Lord in song only to have to listen to a short excursive sermon; or to want to engage in prayer for others only to find that a short discourse on something such as faith or prayer or anything else in the intercessor's mind needs first to be endured.
Secondly, it is rude to the preacher her or himself. Little sermons interspersed through the service undermine the commission of the preacher to be the preacher at that service. Likely they detract from the message the preacher has been given by God.