Tea is a huge subject. The most I can do in a single post is give an introduction, and offer some pointers to places where you can find more in-depth information.
The prices of good Chinese teas can come as a bit of a shock if you're not aware of how to use the leaves efficiently — the last batch of white tea I bought was £6.50 for 25g, which at first glance seems ridiculously expensive. However, like most good loose whole-leaf teas, you don't actually need that much of it to make a good brew, and the leaves can be brewed up to three or four times, with the flavour changing subtly each time.
The most important factors for a good cup of tea are:
- The varietal and quality (grade) of the tea leaves you brew with.
- The temperature of the water.
- The length of brewing time.
Other factors can have an effect too. For example, if you reboil a kettle over and over then the oxygen content of the water decreases, and some people find there's a discernable effect on tea brewed with this water. Tea made with filtered water, or with tap water from different regions, may also taste different. However, assuming you use the same type of water every time and fill your kettle fresh every time, it's the three things above that deserve attention.
The optimal water temperature depends on the tea. Hopefully, either the packaging of the tea or the person you buy it from will provide this information, but a reasonable rule of thumb is 80˚C for green teas such as dragon well and white teas such as silver needle, 85-90˚C for oolong, and 95-100˚C for black teas and pu-er (which is sometimes classified as a green tea and sometimes as a black tea).
Brewing time varies depending on the amount of leaves that you use, and whether they've been infused before — second, third, and fourth infusions require progressively longer times, and while a first brewing may be perfect in only 2-3 minutes, the fourth may take as long as 15 minutes to extract all the flavours. As vatine points out in comments, different teas have different-sized "windows of opportunity", too — with some teas, you have to get the timing very precise, while others are more forgiving.
Note also that not all Chinese teas are actual teas, i.e. infusions made with the leaves of some varietal of Camellia sinensis. Chrysanthemum tea (菊花茶/jú huā chá), which I mentioned on Wednesday, is an infusion of flowers, and hence caffeine-free.
For more information on tea, the Single Estate Tea blog is worth a read, as are the Chinese Tea Files, Life In Teacup, The Mandarin's Tea, 0olong's overview of tea and tea types, and Life On Nanchang Lu's guide to pu-er tea.