For routine use (e.g. grocery shopping) I use a half-inch (or maybe it's three eighths ...I have a cat on my lap right now and can't go look (edit: 5/16)) wire cable and the u-lock from my NY Chain. I loop it around both frame and rear wheel. I also have a thin cable that secures the seat, rear rack, and rear rack basket. That cable wouldn't stop anyone with a cutter, but at least someone can't walk up with a hex wrench and walk away with those parts.
For non-routine use, I carry and use the chain, too. But it weighs about 3KG so I prefer not to lug it around routinely.
As a side note: I, too, dispute the idea that it's the poor who steal. In my experience, people are very often poor precisely because they're honest and non-exploitative. It's the well-off, with their overweening sense of entitlement, who do the most thieving--generally under color of law.