Think back to the last time you saw a good scientific talk. What were the features of that talk? Undoubtedly there were interesting results that you had at least a passing interest in, but is that all you remember about the seminar?
When you go to a conference you spend your day being bombarded with information that is typically at the forefront of our knowledge. It's exhausting and mind-numbing and exciting and overwhelming. The typical conference audience sits for hours each day watching one person after another talk from the stage. Whereas having a topic and data that people find interesting is really important for giving a good talk, so too is your relationship with your audience.
What often separates a good talk from a really memorable talk is keeping the audience engaged. Too many speakers try to do that entirely with their slides, but there's ALWAYS a population of an audience uninterested in the results and a much bigger human element to grab those people than many appreciate. The talks I most remember are ones that not only had interesting data, but where I felt like there was a real person explaining those data.
One of the reasons I never practice talks anymore is because I realized that when I get bored with a presentation I project that attitude to the audience. If you are going through a talk for the tenth time and are simply the vehicle for delivery of the information, you have no hope of pulling in people who are not inherently interested in the results. I see far too many talks by people at all career stages that have all the excitement of reading the congressional record out loud.
Not everyone can make a career on a deadpan delivery.
As you become more and more accustomed to speaking in front of an audience, the most critical thing to work on is using your delivery to engage your audience. A seminar is not a book report punctuated by verbal disfluencies. It's your opportunity to show your audience why what you found is really fucking awesome. You should be excited to lead them down the path and really want to tell them the result. If you can't convey a level of interest in your own work, why should they care?
Don't let stage presence be an under-appreciated part of your development as a speaker.