Invisible packages (as discussed in the related answer) are a big help when you want to improve (or control) to some degree the layout of a diagram with lots of classes and associations.
The problem I found is that if you also want to have some visible packages, it's not obvious how to do that. The skinparam definition sets all packages to have the colors.
I initially tried to do it with a stereotype, as one does with classes, e.g., <<LayoutGroup>> for packages used for grouping purposes. But I see there are no colors associated with Package stereotypes in skinparam.
Perhaps it would be nice to have a new package stereotype called <<LayoutGroup>> which by default has everything hidden, perhaps with skinparams analogous to class stereotypes, which can be redefined in skinparam.
These are just suggestions and I realize they could have implications with other features.
Also, for the record, I think it's better to use Transparent as a color to hide the packages (as opposed to White in my answer above)
skinparam package {
borderColor Transparent
fontColor Transparent
}