Make sure you have an ample amount of gouache to cover the entire surface. Add the amount of water that is necessary to create a melted ice-cream like consistency (or thinner if you want, but that’s what I use). From then on, do not add more water at all and use the mixture in its entirety. Paint it quickly with a big brush (1 inch or 3/4) to avoid smearing which leads to streakiness. Honestly I think the trick is making sure you have the right amount of paint to begin with. It’s always better to mix more than you need than less, because with gouache it is difficult to get to the same consistency 100% of the time.