Loosely-coupled applications are typically implemented using message-oriented architecture. The more loosely coupled the application components are, the better they will scale. Design your application to comprise of independent components. Design everything as a black box and decouple interactions to the extent possible. You can use the AWS SQS service for this purpose. SQS queues are commonly introduced between application components to buffer messages. This ensures that the application is functional under high concurrency, unpredictable loads, and/or load spikes.

Loosely-coupled components enable you to differentially scale out your architecture by deploying more instances of any given component or by provisioning more powerful instances for the components that require it. You can also provision specialized EC2 instances to meet the specific requirements of your components or use cases, for example, computing optimized, memory optimized, and/or storage optimized instances.

In addition, you should try to design your application components to be stateless services, as much as possible. This will help you distribute your components more effectively. With a Service-Oriented Architecture (SOA) you can move services into their own tiers, treat them separately, and scale them independently. This also offers greater flexibility and understanding of each component. In situations where you need to store session states, it is important that you do so outside of your component, so that it is accessible from any instance serving your users' requests. This is especially important in the auto scaling context, where the number of instances are varying in response to demand. 

Loose coupling plus SOA is a winning architectural combination whether on-premises or on the cloud, so spend the time and effort to architect your applications according to their key guiding principles.