Practice the music again and again until you can perform it to the best of your ability. If you are worried about certain sections focus on these in the run-up to the concert.
Tape your practice sessions and listen to them critically to improve your playing. Perform pieces in front of different groups of people so that you get used to performing in front of a live audience.
Sit at the piano at the concert before the audience arrives and play it a little to get familiar with the particular instrument, the seating and the layout of the stage. That way there will be no surprises, which can help reduce the anxiety about stepping onto the stage.
Find a quiet corner before the performance begins to steady your nerves. You may find that breathing techniques and meditation can help calm you. Alternatively, do some exercise, such as fast jumping or stretches, to fire yourself up, get your heart rate going, and reduce performance anxiety.
Avoid other performers who are displaying overt nervousness as this can rattle you. Mix with the confident, laid-back musicians. For some pianists, getting to know the other performers might help them feel relaxed about the performance.
Count to 10 just before going on stage and take regular, deep breaths. Close your eyes to block out everything around you.
Focus on the excitement at performing the piano of the live event. Tell yourself that it will be a great performance. Smile as you walk onto the stage.