There are squillions of tutorials out there, each with their own tips and tricks- (just try a Google or YouTube search for water marble nails). Since discovering the technique I have never really had any problems getting it to work. I think its just luck though!
I have always tried to use filtered water- not sure if this helps, but lots of people suggest it. Maybe one day I should try filtered vs unfiltered to compare. The biggest component of water marbling is finding the right polishes, so that they don't dry before you have a chance to make the design. I don't have any particular trick for this, other than trying different polishes to see which work.
For this design I started with a white base (ChiChi), and used Jezy "Peacock Blue", Sally Hansen "Blue Me Away!", China Glaze "Sea Spray" and a light blue (Jezy "22").
Excuse the slight dings on my index and middle finger- I was doing this after softball training so was tired and not paying enough attention...
I am pretty happy with this colour combo, and I am now itching to do some pastel water marbling!