The perfect cup of tea

How do you prepare a perfect cup of tea? Ofcourse, perfection in this case refers to ones taste. Everyday I use the same quantity of water, the same tea bag, the same amount of sugar (two tea spoons to keep my sweet tooth happy) and the same amount of milk. Looking at it with pride, I walk back to my desk. Some days I finish it off with happiness because it really tastes the way I want it to. Some days (like today) I wear a disgruntled look on my face wondering what went wrong. Why am I facing this terrible torture of having to drink a bad cup of tea? What can I do to ensure a perfect cup of tea when I need it? :(