After reading this question and David's answer I thought a more general question asking for tips would be useful. So
how can authors make (and write) 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.
Have better things to write about. Write about them better.
Stephen King's advice for writers is surprisingly relevant: http://lookingforlola.wordpress.com/2009/05/05/stephen-kings-advice-for-writers/
Concerning technical writing:
Check spell your paper, review your notations (even though an index of notation cannot be included in a conference submission in general, doing it early helps for later, longer versions, and for the clarity of the early versions), have someone else than those involved in the writing to read it and check it is understandable.
Concerning content: Justify all your choices.
When you have several related results, find what they have in common, describe it as a question, and explore alternative answers to this questions that you did not consider before, eventually adding new (minor?) results or justification for discarding alternate answer.
When you have a single result, a single answer to one specific problem, describe what are the other possible answers to this problem, in the literature or based on futuristic results.
Do not limit your paper to "I did this, and look, it works!". Explain why it works and why or how other choices would work worse or would not work.