i have also experienced a loss in flavor over time, sadly this is natural. the Alishan i am drinking right now is in a metalic colored, sealed plastic bag. I keep it at room temperature and away from sunlight in a cabinet. I have found that this will help to preserve the wonderful flavor much longer.
Also, i personally believe that brewing the tea Gong Fu method will yield the best possible flavor. I drink the same tea at work that i drink at home, only at work i do not have a gong fu set, just a Korean crackle ware cup with infuser inside. I must admit that the tea i brew at home with the gong fu set (exact same batch of oolong) tastes much better, and sweeter, every time. At first i thought this might simply be in my mind, but i am now convinced! something about the yixing ware makes a noticable difference in taste.
for your information, the yixing pot i use is about 3 oz. I place just enough leaves to cover the bottom of the pot (this seems to be the right amount for my particular pot). I then use hot water made from my Adagio Utilitea, setting the temperature to the lower end of the green tea setting (the water is probably around 180-190 F) I get perfect tea every time this way, and as i mentioned above, the tea is much sweeter then when i brew the exact same tea using my Korean cup or my Tetsubin. I hope this helps you out!