There are no special methods for Duskborne. But they usually come from sub-optimal aetherial reduction like 2/8 to 3/8 Purity. Hence, they're the easiest to obtain.

Dawnborne require slightly higher Purity, where as Leafborne and Landborne require the highest Purity (usually from 8/8).

I have listed the method for 8/8 in Ch 11 of my gathering guide in case you wanna check it out:
http://forum.square-enix.com/ffxiv/t...Caimie-Tsukino

Unfortunately, the type of aethersand that comes out in the end is still ultimately determined by RNG. Rarity only changes the rate of each type of aethersand, but it never guarantees 100% for any particular kind of sand.