HA AWS RS Overview (Presentation link)
AWS has many features to provide HA/DR.
Presentation attached delivers some ideas.
1 ) Use an ELB, Auto Scale, Cloud Watch integration at the Web HTTP Server and Web Application Server tiers. Stateless data within these 2 tiers are perfect for the HA provided by ELB and Auto Scaling (Horizontal scaling).
2 ) For the DB and persistent stores, DB replication, active-passive node architectures and having a XFC or controller polling the active nodes is mandatory.
Other items to worry about:
Instance costs and Capacity
Replication, Read Only
Active-Passive relationships per node
EBS and DB Snapshots stored in S3