Hehe, it's times like these I really wish signs would stack - you can't bring nearly enough on a single trip to label an entire cavern.
You can bring in a 64-stack of logs. That's a
whole lot of wood in one slot. I tend to drop a small chest and a workbench in convenient locations. Mind you, crafting in a partially-explored cave is dangerous.
You could also install More Stackables and get 8-stacks of signs.
One thing that can help is landmarks. Of course, caves all start to look alike after a while, so identifying natural landmarks is tricky - I've never tried it, but it might be helpful to bring stacks of coloured wool into the caverns, and use them to mark major intersections. Nothing says, "This way home!" like a massive pillar of neon-purple wool.
I do the wool thing on all mining and all but the smallest caves. Yellow next to pink, the pink points to the entrance. Yellow-red, the red points to lava. A single yellow, this area needs to be explored. A single red, dangerous area. (These are all colors craftable from flowers.) A single white, or several of a single color in a row, marks a fully-explored branch.
Before wool, I used planks, sand, and glass as markers. None of those are normally found underground, and they're all distinctive.
I've never liked the torch-on-right-wall rule. I prefer to zig-zag my torches for maximum light coverage. (I have gamma problems with my monitor.)