Both pianists and guitarists need to be aware of their hand span. Since my hand span is only average I avoid doing large chords on the piano and have noticed that using playing in a key that uses the black keys (e.g. e flat) enables me to have a slightly wider hand span.
The little finger, sometimes referred to as the pinky, is the weakest fingers and needs to be used carefully when playing. I tend to avoid holding down other keys when I am using my little finger on anything but a chord.
i) When playing minimise the amount of force you use. Play with a light touch. This will improve your style by making it easier to vary dynamics.
ii) ensure you have the right posture and use an appropriate stool set to the right height. I find that when I am reading music whilst playing a keyboard rather than use the stand that attaches to the keyboard, it is better to use a separate music stand so that I am not stooping to read the music. A stool is better than a chair because it gives you more freedom of movement.
iii) warm up with scales
iii) do not play on a cheap keyboard that has a poor action.
iv) some people advocate special exercises such as ‘spanning’ where you extend your fingers.
v) do not force your hands to play part of pieces that strain your hands. Either skip these bits or modify them.
vi) build up to playing more complex pieces so that your hands gradually get used to the different demands of more complex music.
vii) when learning a piece sometimes it is good to learn the right and left hand parts separately. This way you are giving your other hand a rest when you practice.
viii) more difficult pieces need more repetition, so I prefer to combine learning a difficult piece with easier pieces.
ix) If you are playing on a keyboard try to use a fixed stand or lean it against a wall so that your hands and wrists aren’t absorbing stresses caused by the keyboard moving as you play.
When practicing it is possible to give your hands a break by doing ‘head practice’ where you mentally rehearse what you are going to play.