After killing several batches of water kefir grains, I finally settled on the following technique for feeding my water kefir grains. (We don't drink the liquid, but I use extra grains for culturing watery beverages like coconut water. Meanwhile, the grains stay alive in their primary medium of sugar water.)
1/4 cup water kefir grains
1/4 cup sucanat
1/8 tsp. baking soda
1 slice unjuiced lemon
filtered water to cover, and mostly fill a 1-quart jar, with at least an inch of headroom
I allow this mixture to sit out at room temperature for two days, in order to grow and then harvest the grains. Once I've taken the grains I need, I put the remainder into a new sugar/water solution and place the jar in the fridge until I need it next. Then I just pull it out, and place it in the cabinet for a couple of days, beginning the cycle again.