How many times have you said to yourself or your teacher “I played it better in practice!”
One reason you may not play your best when giving a performance is that you have been practising in a different way to the way you perform. You should therefore make every effort to play in the same way wether you are practising or performing.
So starting with the basics make sure you are sitting or standing in the way you would be for a performance. Make sure your instrument is setup/oiled in the same way. Hold your instrument in the same way. As far as the actual music is concerned you should play everything on the page as you would in performance. That means all the dynamics, expressions, articulations and anything else the music says.
If you do practise in exactly the same way as you perform you will undoubtedly become a better performer.