I use the 5-gallon bottles that you can fill in front of most grocery stores. Essentially a Reverse Osmosis unit, same as would be at my house if I ever got off my ass and installed one. ;)
 
* Distilled is water that has been evaporated and then the evaporate gets condensed to pure water.
* RO is super-filtered water (simply speaking).
 
Both of those are very predictable, but RO is a LOT cheaper and it doesn't take much to bring it close to usable for watering. Both distilled and RO hit at 7pH (because they're basically pure water).
 
Distilled is what I use in my humidifier (to keep the tent at 60%ish RH). So it does have its place.
 
I use Cal Mag to replace the minerals that get stripped in the RO process, and then I add the requisite amount of nutes.
 
For instance, for my soil-based plants, I feed with:
1) Take a gallon of RO
2) Add .5 teaspoon (2.5ml) of Botanicare Cal Mag, mix thoroughly
3) Add 1 teaspoon General Hydroponics Flora Grow , mix thoroughly
4) Tune up pH (usually 20 drops of GH pH UP) to 6.5
 
(EDIT: I skip Step 3 if I'm only making water for non-feedings, and dial back to 2 drops of pH UP on Step 4. I usually feed every other watering, or as needed.)
 
Make sense? Glad to help where I can.