I also have been able to keep mixed water for at least two months. I just keep it closed in 5 gallon buckets with gasketed lids. The night before I use it, I will bring it upto temp and mix it good for 12-24 hours. I have had no troubles with this setup as of yet.
Bob Fenner says in his book that you should "age" your saltwater mix for 1-2 weeks before putting it into the tank anyway if at all possible.
125 Gal-35 Gal Wet/Dry-Tetra-Tec PF500 HOB
(3) AuquaClear 301 Powerheads-600 GPH
Automatic Tempature Controller
5-6 Inch Sand Bed-200+ lbs of LR