I was going to recommend Toastmasters, too. We have a group at work and it's fun! It's people from all round the business, many of whom I wouldn't come across in "normal" work, and we have talked about all sorts - rarely work, but a couple of people have done practice runs of work presentations.
I've also been to a local group as a guest, and it's interesting to see the different ways they operate.
It does spoil you listening to people, though. I find myself silently counting the ums and has and crutch words like so, y'know and the like, and thinking about the structure and so on, rather than the content...
Another thing I did recently was a Franklin Covey course on presentation skills, where we got videoed as we refined our speeches over a couple of days. That was a good course,but I suspect like other courses, a lot depends on the person leading it. Good material only gets you so far, which I guess is partly why you started the thread.
But I'm assuming that if you are giving these sorts of speeches once a month, it is a fairly big organisation, so that sort of training provision might be available. Even if it's not, see if you can get yourself recorded. It can be a horrid experience, but it really does show you how you rock side to side on your feet, or wring your hands together, or talk to the screen, not the audience and so on. Find someone you trust to do it. And then, like everything else, it's a question of practice. (And one day, I will learn to just take a breath without saying um... Actually, I am improving. I just have more improvements to make.)