Few months back I have start a company for that exact purpose : helping you decreasing you AWS bill. https://wizardly.eu/
The focus is around reservation, unused resources and tracking cost evolution on a day to day basics.
I don't get that much traction ( I'm not a marketer and it's my first SaaS project, still a lot to learn in that area). I'm looking for more users feedback, if you willing to invested few minutes in my project that would really helpful. It's also completely free now.
As the article say, their is no silver bullet. Specially if you need infrastructure changed. But it's possible to have an interesting cut by using all the AWS pricing options.
Making teams aware and responsable of their costs is also making a big differences. Often, people creating and managing infrastructure have no view of what it is costing for their company.