Several years when I was teaching a public speaking class, a few of the participants remarked, "we already know this stuff." However, when I watched their presentations later that day, it was clear that whatever their claims about knowing "this stuff," they hadn't put it into practice.
Their presentations were full of filler words, like "um" and "ah," the organization of their information was jumbled and hard to follow, their slides were overcrowded with too many words in tiny font and their body language didn't support their message.
It's not enough to know something intellectually about effective presentation skills – you have to practice it consistently in order for it to become a habit and a normal part of how you present.
For example, it's not enough to know that you should avoid using filler words and pause words such as "um" and "ah" when presenting because too many can interrupt the flow of your ideas and distract the audience. Just knowing that won't help you avoid using them - you have to practice speaking without using them in order to make it a consistent and regular part of your speaking skills.
(An effective way to practice is to count pause words and fillers in other people's presentations and in your own. In fact, Toastmasters, www.toastmasters.org, an international organization dedicated to helping people improve their public speaking skills, has an Um and Ah Counter at each meeting to count each person's pause words and filler words. The goal is not to embarrass you, but to help you become conscious of when you're using them, which is the first step in eliminating them from your presentations.)
Your goal is to master the fundamentals of presentation content and delivery through repeated practice so you can integrate them into how you normally and naturally present. Knowledge is not enough – you have to apply your knowledge. And then you're ready to say "I know this stuff" and "I do it consistently."