in my experience, aculux suffers with the same "sudden death" syndrome as xtol is reported to show, so I reckon it's best to always buy small amounts and use regularly. I daresay that keeping in glass bottles will help, as will removing any air from the bottle by either using glass beads to raise the level, inert gas sprays, or as I do, using vacuum pumps and stoppers designed for use in wine bottles (5.99 in Sainsbury for pump and 3 stoppers - extra stoppers available for about 1.99 for pack of 4)
It is a great developer though.