Practice soloing in every position This is the last step before not having to prepare for the solo at all. But just because you don't need to prepare in advance it doesn't mean you won't benefit enormously if you do. As we grow better and better we also tend to grow more and more comfortable - and we start using the same tools over and over again.
If you want to become better at new things all the time, you must constantly focus on it. Left by itself the brain has a tendency to want to do the same things over and over again. And so we become better at what we're already good at. But this is great because it leads us to mastery. But we need to balance this pull by constantly using tools that we're not super familiar with.