Follow all the tips on the beginner page and most importantly, practise SLOWLY and in a RELAXED manner - whether it's new music for you, or previously learnt music. This helps deep learning and reduces the risk of injury as well as being essential for producing a good sound.
Be careful not to fall into bad habits by becoming overconfident and careless about fingering. If you use different fingerings each time you play a piece, the brain thinks it's a different piece of music and you can't take advantage of the muscular memory you have already developed with hard work.
When you revise old pieces, therefore, be sure to approach them as if for the first time and do hands separate and slow practice of short sections with meticulous attention to the fingering. After enough of this sort of practice, you can get back to performing to yourself and then to your friends.
Your pieces need to be maintained and kept in good shape for you to get the best out of them. Play them at least once a week to keep in touch otherwise they will deteriorate.
Most important is to never stop listening to every note even though you now think you can play the piece. Don't rely on auto pilot even if you are just playing purely for enjoyment. That music has to projected to the listener as well as yourself.