Double your di up. Ive done that and i change mine once a month (300gpd).
Start off with one cartridge, when it starts rising (i go to 005ppm) you put the second cartridge with the new resin between your first and water storage.
I changed mine over the other week and im not expecting to do it again now till mid April.
The shake method works too, but my system is all hooked together with bolts, so i found that a pain in the ass to do myself.