During your training is important that you have everything in place to keep your focus and maximise the training benefits - technical tips, correct equipment, following the program with right intensity and commitment, and so on.
However, one element is paramount, your hydration!!! Swimmers can lose more than 1 litre of fluids during one hour of swimming training. To perform well, you need to train well - to train well, you need to be well rested and hydrated!!! Have a look at this great article on hydration for swimmers.