It is easier to be passionate about your work - and the presentations you make - if you are doing what you love to do. Kathy Sierra has an interesting post about how to redefine success in terms of doing what you want to do.
I recently gave a talk to some new teaching assistants on what works for me as a teacher. I believe that you have to love not only your own subject, but also the art of teaching, and your students themselves. I think this applies to any type of work: love your industry (e.g. software), your function (e.g. marketing), and your clients/customers. In the latter case, sometimes your mind has to lead your feelings here: you have to want to love them first, even if they do not appear to be very loveable initially. But if you care enough about helping them, you will come to love them over time.
I have found that reflecting on these three aspects of my career (what I'm doing, where I'm doing it, and for whom I'm doing it) has paid big dividends over the years. After making a number of mid-course corrections (quitting McKinsey & Co. at age 30 to study theology; returning and then quitting again to do a Ph.D. in marketing and ethics 3 years later), I am now 41 and doing what I love: teaching marketing, consulting to great companies, and spending lots of time with my family.
I won't say it's easy; there some are serious tradeoffs to make. (Paul Graham has some suggestions about how to deal with these). But I think it's worthwhile to spend the time reflecting on what you'd like to do, where you'd like to do it, and for whom.