Updated: Jul 28, 2020
Stage composure is a broad topic, so I will stick to things pertinent to performing
Recognize that you will make mistakes! In a way, this is liberating, because every thought you spend dwelling on 100% perfection is a thought not spent on playing (though 99% is still a nice goal to aim for!). If you make a mistake, keep on playing, do not apologize to yourself or the audience, and keep on going!!
Realize everyone is probably just as nervous as you! Performing is not easy, so don't worry about making it seem like it's not. If you're worried how others might see you, keep in mind they're probably more focused on themselves.
Practice making the mistakes you wouldn't normally have! This one I draw from personal experience - my sweaty hands and worrying/senility over tricky technical issues would always give me trouble performing; thus during practice, I would close my eyes, point to a passage, and play. You can't prepare for everything, but practicing memory faults and starting in strange areas will help.
Last of all, be confident and enjoy yourself! I know this may seem sappy, but you really must make your mind your ally - or it will become your greatest enemy. If you can "fake it till you make it", your confidence and comfort while performing will smooth out those shaky hands and butterflies in your stomach.