People are not listening all the time so introduce important points with the right words, such as “This is vital” or “Please listen carefully”. People will not know what’s important until you tell them.
If possible, also raise your voice, speak more slowly, lift your hand, walk towards your listener or simply stand up. Movement always attracts attention.
Also, if part of your speech is directed at a group, let them know. Imagine talking to doctors and nurses. By saying “Now I’ll speak primarily to the nurses in this room” you’ll allow the doctors to drift off and be ready for the moment when you need their attention