Two words: billing alarms. If you have stuff on AWS, and you don't have billing alarms for actual and predicted costs, you have nobody to blame but yourself for extra charges.
EDIT>> I see they address billing alarms but claim that they only fire after you've lost the money. This is simply not true with the predictive alarms. I get the sense that this author is not using AWS correctly in general.
When I was initially perusing AWS at my current company I was able to save us a ton of (recurring) money simply by properly deleting EBS volumes of terminated machines... someone didn't check a checkbox at some point (and/or didn't understand its significance): oopsie.
It's hard to set alarms early on when you're building stuff out. In any case I think it really needs to be someone's role to understand the billing thoroughly, somewhat regularly. Our billing line items post daily to me on slack, mostly I'll just check the total vs the previous week, and cloudability sends email alarms for specific things and suggests RIs etc.
Their billing cost explorer tool is fantastic. It will show you exactly what services are costing you, broken down by their usage pattern.
And that's if you even have a budget :)