To mention just the first few that come to mind:
- Setup aws billing cost alerts based on thresholds
- Consult AWS billing dashboard daily first, then weekly, then monthly. There is a "forecast" that can tell where you're going to land at the end of the month (ballpark).
- Understand how s3 charges, take advantage of the various storage classes. Setup s3 bucket lifecycle to delete objects if you have expiring objects.
- Make sure you setup backups. This can be a source-code repo or something like glacier. Make sure you understand how it works. tl;dr archiving is cheap, retrieving is expensive & slow.
- Make sure you understand how data transfer is charged on AWS (e.g. https://aws.amazon.com/blogs/architecture/overview-of-data-t... - there are many others)
- Avoid services that use the AWS API to fetch metrics unless you have bandwidth to monitor closely. I've seen companies getting charged 6-7k/month for months, without anyone noticing, just because a prom exporter was abusing the API. When I took notice, three months in, I removed the exporter without letting anyone know. Scream-test was passed in flying colours, no one complained :-)
Good luck! :D