Me, I normally put a cup of sand in with each cup of professional potting mix (bunning type) because I believe that it is not sandy enough and too general.
Sometimes I throw in 10% pertlite too for aeration. With Astros I use 70% sand.
Then I normally put a layer of fine sand (for Lithops or tiny seed) or course sand for astros and arios and the like.
It depends on the seed and plant type.
You can really go into it with granite chips, limestone and all sorts of mix to mimic the natural conditions of the seed. Depends on how much effort you want to put in, it can get awkward, but the more rare and difficult the seed, the more you should mimic all conditions.