To prepare, play through your piece once stopping to mark with a pencil any tricky bits.
When you practise play only those tricky bits you marked with a pencil. Practise until you have mastered all of those bits. You are not allowed to practise any of the easier unmarked sections.
Once you can play all those tricky sections go back and rub out your pencil marks. Now play through the whole piece again marking the tricky bits – they will be different this time!
Once you have repeated this a couple of times you should find that there are no sections left that are harder than others.