What did you do differently on your best launch compared to your worst one?

I've been reading a lot of launch retrospectives lately and the interesting ones are never about the standard playbook (email list, communities, replying to comments, we all know that part). The good stories are always something specific the founder figured out for their own product, like one who spent the week before launch in the forums stress-testing his premise, so by launch day half the commenters already knew the product and had context.

If you've launched here more than once, what did you actually change between your launches, and which change made a real difference? Just as useful, what did you double down on that turned out to not matter at all?