My favorite that most people miss is the S3 VPC endpoint. Putting an S3 endpoint in your VPC gives any traffic to S3 it's own internal route, so it's not billed like public traffic. I've seen several reductions in the 20-50k a month range with this one trick.
Otherwise, stop doing baremetal processes on the cloud. It's dumb. For instance, I see people operating a file gateway server (with 5 figure a year licensing costs) on ec2 using EBS storage. This is a perfect use case for replacement with S3, with lambda code running in response to whatever file gateway event you're monitoring.
Lastly, you need to really challenge people's conceptions about when resources need to be in use. Does a dev/test site need to be running the 16 hours that people are not at work? Of course not, except when people are staying late. So you create incentives to turn it off, or you run it on something like ECS or kubernetes with software to stop containers if they're not actively used in a 15 minute window (and then the cluster can scale down).