Here are a few more tips to help with your cost-optimization efforts:

• Services that launch instances on your behalf (auto scaling, Elastic Beanstalk, and so on) are also eligible to have hourly discounts applied to them.
• Standard reservations can be made for either 1 or 3 years. As mentioned before, Convertible reservations are fixed at 3 years.
• Under a consolidated billing model, reserved instance discounts are applied across all your subaccounts. For example, if you purchase a reserved instance intended for account A but there is no server matching its properties, the reservation will automatically apply to matching instances in account B. This only applies to the billing benefit and not the capacity benefit. Note that this behavior can be disabled.
• Reserved instances can be sold in the AWS marketplace. This is useful if the reservation no longer suits your needs. Note that you will need a US bank account for this.
• If reserved instances don't seem to match your type of workload, you might consider a scheduled instance or a spot instance instead.