Part of it is building a team over a few seasons that grows together. Throwing money at promotion doesn't always work well, though it did in Wolves' case. Swansea are a perfect example of that. They improved what they could bit by bit and the players were good enough when they won promotion to not only stay up, but to do quite well initially.
That's why I think that had we won promotion a decade ago, we'd have been ok. We had some good footballers who would have been fine in the top flight that had played with each other for a few years. Age was on our side in many cases as well.