I seemed to have done reasonably well with two approaches. One, I am very dogmatic. This doesn't take a lot of thinking. If you want to prepare something, write down all of the possibilities. If there is a bond you wish to make, there are three ways to do it, +-, -+, and **. If you begin to see which ones actually seem reasonable, some are a lot better than others. I also used this as a reaction searching strategy, either to find new chemistry or create it. Umpulong strategies fall out naturally.
The other part we can call, "look it up". It is one thing to theorize reactions, but your molecules will undoubtedly have context. There have been other similar reactions that have been tried in the past. It is helpful to know which chemistry is good chemistry and which is a prayer that you hope will be answered. If you are betting your career on one reaction, I'd avoid hoping your prayer will be answered.
When I was in industry, we had REACCS. What was good about it was that it was a filtered source of reactions. In my case, by being dogmatic, I wasn't afraid to strip away a lot of chemistry to try to find new reactions. That sometimes resulted in some pretty large hit lists. I'd look through all of them by-passing the familiar or similar looking for the unusual. All of this should prove to you that I used perseverance in lieu of intelligence.
That was my approach to create out of the box thinking.