One of those things is learning which polishes will marble and which ones will not. Some will always marble (for example, OPI Color Paints which I can't stop using), some will but not with certain other ones, and some just will not work. So experimenting with it from time to time certainly helps.
Another thing that helped is getting the actual technique down. Practice makes perfect, as they say. Learning how to create different marbled designs makes it a breeze in the end.
The last helpful hint I learned from watching Colette from My Simple Little Pleasures on YouTube. When water marbling, she will sometimes dip more than one finger at a time into the design. WHAT A REVELATION! The first time I saw her do it, I though, "Duh! Why didn't I think of this?!" It saves time and nail polish. It does take some practice, though.
On to the nails...
Naturally, I used the OPI Color Paints for this design. I used Silver Canvas for the base and marbled with Magenta Muse, Chromatic Orange, Primarily Yellow, Landscape Artist, Indigo Motif, and Purple Perspective. To add some holo awesomeness, I topped it with FUN Lacquer Diamond Holo Top Coat.