Always use freshly drawn water or newly opened bottled water to make your tea.
Don’t use water that has been sitting out for hours on end. When you start boiling your water don’t walk away if, for example, you are boiling water for delicate teas such as green teas. Delicate teas need continuous monitoring. Over-boiling your water will result in too much removal of oxygen resulting in a ‘flat’ cup of tea. A flat cup of tea is caused by a lack of tannins. Tannins are what give tea its astringency, a desirable trait in tea. Also, never re-boil your tea. You will most certainly have an even flatter tea!