The best way to pitch to a client is to know what you're talking about. So the biggest thing is knowing exactly the key points that they wanted you to solve, all the problems that they had, all the questions that they like sent over to you late in the night, and then also being able to visually display how you understand and translated everything that they gave to you into a presentation. So pitching an idea can be anything from pitching a concept, whether it's hey, you want to do a commercial or you want to do a website and you want to have three different people do this website and pick your favorite one, all the way up to, "Hey, we've finished your project and now we want to pitch it to you and show it to you through our eyes."
The worst thing that you could ever do is just send something over to a client without any explanation whatsoever and then just ask, "Do you like it?" You want to make sure that you become one with the project, become one with the presentation, and then also ask, "Hey, can I go over this with you as you look through it? Let me tell you why I felt this style," or "Let me tell you why I chose to do this" or "Why I decided to do that." This is important because without any of these explanations, the client's interpretations can get lost very easily and so can what you wanted them to take away.
So the best way to do a presentation is really just to make sure you outline everything that you need to show and show it the best way that you feel comfortable, whether it's through a PDF that's interactive, or not, or whether it's through Keynote or a PowerPoint presentation and making sure that it's not your typical Keynote and PowerPoint presentation, that it just looked like you popped up a theme and put some words in it.
There's a lot of great resources out there with templates that are already designed or that you can design on your own. And then last, but not least, and certainly not the last thing that you can do is a video presentation, whether you're doing it through After Effects or even just talking into a camera and telling the person, "Hey, this is what I want to do. This is my pitch and what do you think?"
At the end of it you really want to make sure that your needs have been met, the client's needs have been met, and both of your expectations are met as well. Always make sure that you get all the questions from the client and that you also make sure that you get all of your questions out so that any iterations that come about can be answered in your next presentation. So, it's all about communication.