A pinch of salt
I am inexperienced at using salt and have not been very impressed with the results on my few attempts. My early practice images are typical. I tried different salts, different colour blends, different watercolour brands, pans, tubes and different papers and got the same nondescript results.
Then I started adding a lot more water to the paper and to the paint and saw a definite improvement.
I was very pleased with the background for my butterfly, but didn't have a straw and found that a bristle protector from a paintbrush is a poor substitute. I found a set of straws in a shop recently, so have four shiny metal straws waiting to be used on something soon.
I liked the negative painting effect, but suspect I could have managed the water better to get a softer gradation of the colour around the butterflies. Something to aim for next time.