I rarely see a presentation that makes me feel that I didn't waste time. There are many ways to screw up a presentation, and it is not because there isn't information available about how to do it right. The main reason is that people consider it as a chore and don't spend enough time.
When preparing a presentation, my main motivation - independent of the topic - is public relations, i.e. presenting provides me with a chance to give people reason to like me. Once people have a positive opinion of me, it will be easier in the future when I need their help or need to convince them about something they don't agree with.
That's why I usually spend a lot of time for preparations. My method is similar to writing a technical report. I start early (a few days to a few months) and iterate on the way, sometimes changing the topic completely if it doesn't come out as interesting as it sounded in the beginning.
So, seek out opportunities to present and spend time on your presentations, it is an investment into your future.