There is no magic formula. We just need a backroom team that does its homework more and actually thinks about signings in the context of the current squad. Some of them still won't land, but as long as enough of them do that the failures aren't catastrophic to the team's chances and don't end up staying at the club for their entire careers by accident, that's the main thing.
I would argue that the only convincingly "good" signings we have made in the last decade are Shaw (easily the best - the fact that anyone is going in on him here is frankly ludicrous), Bruno, Zlatan, Herrera, Blind and (perhaps more arguably) Mata. Jury still out on Martinez, Garnacho and a few others (Garnacho could probably already be considered a successful signings at a club you could trust to flip someone if they need to rather than extending 5 times on unnecessarily huge salaries).
Everyone else has been somewhere between about passable on balance and absolutely dreadful.