I remember how terrified I was feeling when I presented my research work for the first time to an audience. Over the past years though, and after many presentations, I feel that I have gained a certain level of confidence, which seems to radiate off all more senior PhD students. I notice the same confidence when I see other PhD students that have been around for a longer time present their research (students in their 3rd year seem to gain this inertia, but correct me if you'd think my observation is wrong). I noticed it for the first time on the PhD students senior to me, when I was thinking to myself that their stories and presentations sounded so coherent to me, and they gave me the impression that they indeed are the person who knows all the ins and outs of their precise little field of study.
To determine for myself if, or if not, I have reached the point where other people think I am telling them a solid story is hard to assess, but I can give you an insight on what have been the decisive moments in gaining confidence in what I am actually doing.
1. Practice presenting
The best way to get used to giving presentations, is by presenting often. I still prefer to start preparing a presentation a long time before the actual date of the presentation, and then tinkering with the slides a few times before giving the presentation, trying to improve it bit by bit. However, if I hear 4 days in advance that I'm scheduled to give 2 "new" presentations (that is, showing information I haven't presented previously), I won't go into paralyzed panic-mode, instead I'll just make the slides, practice and give it my best shot.
The general idea is that every minute of public speaking requires an hour of preparation, but I'd like to think of those hours as the hours I've put into my research - and then I'm more than fully covered to go out and talk.
2. Practice writing
If I now look at the very first papers I ever wrote 2 years ago, I see something that I would do totally different if I were to repeat it all over again. However, by writing as much as possible, and by trying to write for different audiences, I feel that I have improved visibly, and seeing my work visibly evolute, has as well made me gain more confidence (and more speed) in putting papers together.
3. Check your ideas with researchers from outside your research group
Test your assumptions - and see if they are generally accepted. Talk about your research as much as possible to others, and learn from their questions. Whatever might seem obvious to you when you're looking at it all the time, might need further explanation for someone else.
4. Befriend your data
It's your responsibility to know your data, your experiments, your sample population, your questionnaire results or whatever you studied to the core. I notice that by knowing very well the ins and outs of my test results, and by pulling out examples whenever necessary, I have a unique tool to reply certain questions (in fact, as a PhD student, you're the closest to this information, and the person who has absorbed your own work to the fullest). Take full benefit of that.
5. Reread the seminal papers in your field - and reread them again
It's like being the smart kid who can quote from all the classics in literature, philosophy and poetry - make sure you know the seminal works in your field. No further explanation needed - and no excuses, just make sure you do your homework as you're supposed to.