Say you are speaking for an hour to an audience of 100.  Its just a fact of human nature that nobody in the audience is going to be paying close attention to what you are saying for more than 1/4 of the time.  The other 45 minutes of the time people will be thinking, talking, or just daydreaming. You must accept this as an unavoidable constraint.

Absent any intervention on your part then you will get a randomly selected 15 minutes of attention from each member of the audience.  This means that at any one point in time you will have the attention of only 1/4 of your audience or 25 out of the 100 people.  The very important things you will have to say will be processed and potentially remembered by 1/4 of your audience, the same fraction that will be paying attention to the least important things you have to say.

So what you should do to prepare is ask yourself what are the three important things you have to say and you want remembered.  Each of them should take you five minutes to say.  Then imagine you have a sign that will flash above you which tells everyone in the audience whether now is the time to be paying close attention or now is an opportunity to doze off.  With that sign you could coordinate their attention so that all of them are listening during the same 15 minutes, those 15 minutes when you will be saying your three important things.

Now you probably won’t be bringing that sign with you.  But you can achieve the same effect by using the way that you stand, the way that you talk, and the style of your slides.  When you are saying something important you speak slowly and loudly and you walk up and down the room and make eye contact and your slides have just one or two things on them so that they are easy to read and process.

You are telling them with your demeanor that now is the time to listen.  Later, when you are saying something less important you lower your voice, go faster, stand still and read off your busy slides. You are doing these things to tell your audience that now is the time to think, talk or doodle and rest up for the next important moment.