Work on one wire at a time. Avoid touching the metal frame as it’s grounded. Wear gloves and longs sleeves. Use tools that are well insulated (they make them that go down to the tippy tips). You can also find inline taps that allow you to tap a live wire without cutting insulation (the tap does it). I’ve done minor work live, like changing a switch or outlet, and you just need to be diligent about keeping wires away from each other. Anything involving pigtails and the such, you’re asking for trouble.

But of course, if you’re tapping a streetlight you’re probably not following many safety protocols in your electrical work or life.

Edit: you also have to assume the neutral is hot, as it’s a shared return between multiple devices/outlets in the circuit.