Making an impact on an audience is difficult. Powerpoint presentation design, tone of voice, content, lighting, manner when speaking – it all needs to be thought of when you make a public presentation. The speech needs to be refined and rehearsed.
However, more important is what you have done in the years and decades leading up to that point. If you don’t have the message clarified over time due to a consistent history with the subject, then it will all sound a little desperate. I think you need to have a long track record behind you and the impact you make with a speech is not the result of a couple of nights’ work – rather it is the culmination of many bits of previous work. If you gain a reputation for a line of work that is channeled, focussed and weighty due to your reputation with the audience – you will probably make an impact. If a speech is grappling to be relevant and consistent with who you really are then it will fall on deaf ears.