My technique on using xtol and d76 is to collect about 20-30 undeveloped rolls (like when coming back from vacation). I then mix the powder and develop all the films within a week. This way I'm sure the developer is fresh. That's the only way I can be sure that i'm working with quality fresh chemistry.
I had Xtol go bad on me within days but I have to be honest and confess that it was stored in a half-full container. I had to print those negs on very hard paper. Still, the grain structure was very nice.