What strategies are you using for cloud cost control?

- Visibility of usage and associated cost - using tools and monitoring to meter projected costs
- Regular planned review to identify unused instances that can be cancelled/repurposed.
- Performance review to understand utilization and perfoming optimization 
- Vendor strategy to work in a hybrid cloud model to facilitate competition for new cloud requirements

Ensure a sound contract (expert/lawyer to review your service contract ) in place with your service provider, spelled the statement of work in details, include precise cost structure, penalties, and no vague or subject terms.  Implement a cloud cost management tool to monitor usage and optimized and right sizing resources. Sometimes you costs go up significantly with little support things, like a call for asking a question or getting a simple report.

Constant monitoring, but also constant communication with the cloud provider's relationship manager, to strike deals whenever possible

We need to control our cloud costs (eg memory, storage, traffic, etc.) and optimize regularly to reduce spend whenever possible.

Multiple native application & external SaaS application can be used to monitor cloud usage & can help to control Capex/Opex. It also gives you an alarm if its going to touch the threshold of defined amount. Also, you can plan to remove the unnecessary workloads/apps if not in use.