Water

Water quality is of utmost importance.  Most of the problems caused by hardness or chlorination in tap water can be overcome with the simple use of a water filter. 

In Imperial China, the best tea was made with water from a spring which emerged near to where the tea bushes grew.  If you want to use spring water bear in mind that it should be neither too hard nor too soft, as waters with the wrong balance or concentration of minerals will interfere with the complex flavours and delicate aromatics of good tea.  Suitable spring water may contain 30-60 mg of dry residue per litre. 

This is only a rough guide as the only way to determine a suitable spring water for a particular tea is to experiment - some spring waters produce poor results due to the balance of minerals they contain.  Distilled water is also unsuitable as the complete lack of minerals produces a completely flat taste. 

Water Temperature

Different teas require different water temperature, and initially at least, it may be helpful to use a thermometer or simply allow your water to boil and wait for it to cool for 5 minutes for 90 degrees and 10 minutes for 70 degrees.

Green and white tea - 60-70 degrees centigrade;  traditionally, "cool enough to pour over your palm"
Light oolong - not more than 80 degrees centigrade (Orchid Oolong, Baozhong, Lishan)
Dark oolong - 90 degrees centigrade
Black tea - 90-95 degrees centigrade
Pure tea - 90-95 degrees centigrade