Propagate tags to secondary resources, like EBS volumes. Separate out production from your dev environment-- preferably with a second AWS account with consolidated billing. Buy in to something like Terraform or CloudFormation for building the non-instance parts of your stack.

As you grow, start adding project tags to things. Automatically tag resources with the IAM user who creates them. Have an "untagged resources report" that goes out on a schedule.

Build with an eye towards "when someone freaks out in a year about the bill, what questions will they have, and how can I best position Future Me to answer them?"