Reserved Instances

Use reserved instances to save big $$$.

Reserving an instance is just putting some money upfront in order to get a lower hourly rate. It ends up being a lot cheaper than an on-demand instance would cost. So if you know you're going to be keeping an instance around for 1 or 3 years, it's well worth reserving them. Reserved instances are a purely logical concept in AWS, you don't assign a specific instance to be reserved, but rather just specify the type and size, and any instances that match the criteria will get the lower price.