I'm cheap. I used a pet food storage container from the container store.
http://www.containerstore.com/s/kitchen/food-storage/pet/pet-food-containers/123d?productId=10006457
The 22-lb one holds about 14-15 gallons and usually lasts my 215-gallon for about a week. I currently have it hooked up directly to my RO/DI unit and ran the tubing through the wall of my garage, above a door frame, and into my tank stand. Most people don't notice it at all.
Before I had it hooked up directly to the RO/DI system, I just wheeled it over to my RO/DI setup in the garage and filled it up and then wheeled it back to my tank and put it back in place on the side of the tank. That's why I got the one with wheels.
I thought about getting an electronic solenoid for the RO/DI line like Dan but it caused me to go through my DI resin way faster than I wanted. Everytime that RO/DI line came online, it would push about 120 TDS into my DI resin and my resin wouldn't last past 6 months. I manually flush my line prior to DI stage now for a whole 1 minute and it reduces it to 11 TDS before I allow it to go to my DI resin and that allows my DI resin to last a full year now.
Maybe one day I'll setup a dual electric solenoid setup where it flushes my line for 1 minute first before opening the line to start filling up my ATO. That way, it's like an automatic flush and fill... with the Apex, the possibilities are endless! Dan, you try it first so I can just copy you!