Yeah, masonry sand is always what I was told to use. You just have to be careful not to add it to clay soil or you can make your own hardpan.
I'm surprised no one has mentioned Perlite - it is kind of double duty - helps with the drainage but still keeps some moisture without exposing roots to actual water. I've been using it (successfully, at 100%) to root cactus that I've had rot in a light soil mix. Something else that I'm seeing more of is lava rock.
I'd be a bit nervous about the shells, too - alkaline soils are okay for a lot of C&S, but if it was harvested from the ocean, the salt intrinsic to it will kill most plants.
