Be mindful and observant with the wetness of the shape. If the shape is drying pass a certain time, it's best to leave it along until it is completely dry. If you put wet paint during that stage, it's going to explode and creates “cauliflower edge”. And you don't want that if you want to keep it clean. And that means to leave the painting as is even when it didn't turn out exactly the way it wants. It is better to let it paints itself instead of trying to fix it and making it worse!
One technique I use is to soften the edge with clean water. What I do essentially is to extend the boundary of the wet area with clean water, so that the color inside will spread, but it won't reaches as far, thus creates a soft edge. This is very useful if you want to paint something with both hard edge and soft edge. Such as distance buildings, some soft shadows.
More than 1/2 of the painting is done during the wet on wet stage. Although at the end people tend to focus on the fine detail, what makes the detail works so great is the beautiful washes underneath. Remember water is the limit! within that area, magic happens! Enjoy this stage.