The most obvious method is the method employed in many churches which do not follow a strict liturgical calendar in which they offer a free-text sermon in the morning (that is, one whose text is chosen by the preacher) and a confessional or catechism sermon in the evening. This is certainly a way to enhance the theological acumen of one’s congregation.
Indeed the Westminster Larger Catechism was written as an aid to preachers and this is why others, such as the Heidelberg Catechism, were divided into “Lord’s Days.” The idea was that a preacher would go through the whole thing in one year and then start over. But this has its drawbacks, not the least of which is that unless one is a very creative and dynamic preacher, even the most hardy churchgoer can become bored after hearing the same sermons over and over again year after year.
Confessions have as their purpose to explain clearly the doctrinal standards of a certain group or theological community. They are not to serve as a replacement for the thorough understanding of the Scriptures. In fact, this is the most decisive argument against preaching directly from catechisms and confessions.
Our mandate is to preach the Word. To resort instead to expounding a human document is to confuse our people by blurring the distinction between what is normative revelation and what is to be judged by that revelation. Even when creeds are inerrant (a claim that can be made for the Apostle’s Creed, for example), their proportion, balance, and selection of topics will not be that of Scripture. Furthermore, confessions and catechisms present doctrine abstracted from its existential context – the life-situation of Scripture – and thus obscure its practical relevance or tempt us not to apply it at all.
Also, the use of creeds and confessions provide a good indication of the relative importance of doctrines. As the old saying goes, “The main thing is to keep the main thing the main thing.” Every Christian can think of some preacher who is always preaching on election, or Christian liberty, or eschatology to the exclusion of other important doctrines.
The preacher who is diligent about using confessions as a guide will avoid the pitfalls of constantly preaching his pet doctrines, constantly preaching his favorite texts, and failing to do justice to the core doctrines of the Christian faith.
 Donald Macleod, “Preaching and Systematic Theology,” in The Preacher and Preaching, ed. Samuel T. Logan, (Phillipsburg: Presbyterian and Reformed Publishing Company, 1986), 269.
 See Macleod, “Preaching and Systematic Theology,” 266-270 for a fuller discussion of the use of creeds and confessions in preaching.