How to make a paper stronger?
This is sort of an unserious answer in that it's not what I consciously do most of the time, but:

Think about what the next paper after yours might be — not the next paper that takes some small follow-on problem from your paper and solves that too, but the next paper that takes it to another level, finds tight upper and lower bounds, shows that it's an instance of a more general phenomenon, and generally makes your paper obsolete. Then write about the results in that paper instead of the ones you already have.