Like with composting, it's easy to find grey water systems that are complicated and expensive, requiring retrofitting your plumbing.
Here at Chez Frugal Culture, our grey water system is a bucket.
We keep it on the kitchen floor by the sink. As we wash dishes, we pour waste water into the bucket. Sometimes, to catch more, we keep a big metal bowl in the sink to collect the water as we rinse, and empty that into the bucket.
When the bucket gets full, we carry it outside and dump it on whatever tree or bush looks thirstiest.
It's easy to imagine systems that would capture more grey water, but not so easy to imagine anything easier to implement.
California's been in a drought, and water prices keep going up. Every little bit helps.