Hydration is a pretty obvious issue, everyone has a water bottle in class and it's a nightmare making it through when you've forgotten it. Even so it's easy to become dehydrated without realising it, and a tiny degree of dehdration can have a significant effect on your performance. Bare in mind your hydration needs are different depending on your training load. If you're just doing a couple of hours, water will do. However if you're in classes and rehearsals all day, you need to recognise you're going to be sweating all day, and that you need to replenish both your fluid and salt levels. If you don't want, don't like, or can't face spending the money on sports drinks, you can easily make your own by adding diluting juice and a quarter teaspoon of salt to your water bottle.
Also, remember anything caffeinated dehydrates, so 5 cups of coffee and one bottle of water aren't going to cut it, even if you feel like you've been drinking all day. If you're dancing full-time or close to full-time, try to drink 2 litres of water throughout your training day to stay hydrated. That excludes whatever you drink at breakfast and after you finish at the end of the day.
Just because it's obvious, don't neglect to think about it, make sure you're drinking enough.