Pull a string alongside the new cable, too, and tie it off on both ends. Always leave a pull string in the conduit.

> a string

At this point, multiple. Just in case.

You forgot to also tie another piece of string to the new cable so that you pull the new cable AND this other bit of string through.

This gives you a piece of string in the conduit run to be able to pull through the next thing days/weeks/years later.

> [...] a string [...]

Does someone have a recommendation for a specific material the string should be made of?

Back in my day the local telephone company used waxed lacing cable for that sort of thing[1]. These days it seems that polypropylene string is popular (search on "conduit pull string").

You basically want something that is slippery and will tend to not get stuck. I have used Dacron fishing line, but that is mostly because I had a bunch of it laying around.

[1] https://en.wikipedia.org/wiki/Cable_lacing

They also make bottles of cable pulling lubricant for this purpose.

AKA boy butter [0].

0: http://www.electricalslang.com/Slang/boy%20butter

It’s really overpriced. Sex lube and veterinary lube are the same stuff and much cheaper - you can get it powdered and mix your own.

The sell pulling string in the electrical section of the hardware store.

But make sure you don't buy pushing string by mistake!