I am planning a Show HN (hoping it is a hit) of a AWS hosted SaaS. What measures should I take to support all the traffic from HN?
That said, there are common things that can be done, many of which have already been mentioned.
For AWS, make sure that you have at least two EC2 instances running, and they're configured to sit behind an elastic load balancer, then make sure that autoscaling has been set up.
As dirktheman says, Make sure that you're sitting behind Cloudfront for caching benefits. Also, make sure that your static files are all being hosted appropriately on a static server (Nginx, Apache, etc) and not by the application server.
Consider implementing memcache. This may or may not fit every applications, but if you have some common queries that are likely to be hit during a traffic spurt, having them resident in memory takes some of the load off of the database, if there is one.
Assuming RDS, definitely be using Multi-AZ instances, and also consider setting up read replicas. If your application is more likely to be read than written to, read replicas are an easy way to ensure that the application will still work for those writing to it (generally, paying customers) while mitigating traffic spikes typically common to media blitzes. Also, having an architecture that supports the easy addition of read replicas affords greater scale-up-ability.
Above and beyond that, everything's pretty application-specific. If you wrote the app in Rails, try to find a Rails-production-ready checklist. Same thing if Django, Java, whatever.
Good luck.