- Cache the hell out of everything.
- Fix links to broken JS includes. On http://store.wonderwarp.com you link to https://store.wonderwarp.com/mint/?js which does not exist and wastes valuable processing time only to error out.
- Pack and minimize your CSS/JS files, even just temporarily. If you are dead sure this is going to be a lot of people, every small bit of saved bytes helps!
That's all I can think of since you've already moved the downloads to S3. Your site is mostly heavy on the static content anyways so not much you can do unless you stripped it down, but I would say keep it as it is because it looks great.
Regardless, if your site does go down think of it as a victory. You got so much traffic (and probably sales) to knock you down that you can only become better for the next time you get press. If it's a good product, it will happen again and again :)