At the end of the day the rule is that there are no rules.: you use whatever works. How does that help you solve new problems? Well, you try what has worked before and if nothing does, then you need to play with different substitutions to see what works.
That isn't much of a helpful answer, but you can always post problems (as you have been doing) and someone will give you an idea to try. The best way to learn how to find effective substitutions is to work a lot of problems and gain experience in what works when and why and what doesn't.