Deep preparation is the best way to handle stage fright.
Then, zen meditation. It's not about you. Become the music.
Still, a very great artist I turned pages for once in a chamber concert was shaking so hard I couldn't believe his hands were finding the right notes.
But he didn't make a mistake.
I have the shaking hands problem, but I end up making tons of mistakes. I played Rachmaninoff's Prelude op. 23/5 for my school the second to last day of high school, the first time I had played for the school since 6th grade. My hands were shaking so badly, when I got to the top of the second page (in the Boosey & Hawkes), the second theme of the march section, instead of sounding like a beautiful fanfare sounded more like Schoenberg! It didn't help that the piano I was playing on had such heavy key weight that I could barely push them down...Thankfully, when I reached the middle section, I managed to calm down and play it quite beautifully, and the second time around the march, I was able to play it as close to flawlessly as I ever have. I even nailed the descending octaves after the fanfare, something I had been struggling to do even when practicing alone. The adrenaline did sorta hit me hard though, and I ended up playing the final quiet ascension of the piece at about 150 bpm instead of the 100-108 I had played the rest of the piece at, and probably too loud too, although I don't know for sure...All in all, the piece went about as bad as I ever could've predicted, yet I was a bit of an icon for the school for the last two days of class. I think people just liked it because it was fast and loud and exciting, which, now that I think about it, aren't terrible reasons to like something

Talking about that was rather therapeutic. I've just got to say, knowing how nervous these other performers get makes me feel better about my own issues. If these pianists can have as bad of stage fright as I have, or worse, and still play a hundred times better than me, I have no excuse. I just need to prepare more.
If all that doesn't help and you have physical symptoms of anxiety i.e. sweaty hands, see a doctor about beta blockers. They are not going to make you play better, nor help you be prepared better. They are going to enable you to realize your potential and perform to the standard you have prepared yourself.
I've thought about using beta blockers. It's really tempting, but I'd like to solve the problem instead of just covering it up. I don't know if I'll ever be able to solve it completely, but it's something to work for. It seems a bad idea, to me, to jump to beta blockers as an easy solution, instead of waiting to use them as a last resort.
I guess a gulp of whiskey would work too, you know, "to take the edge off".