If you want to avoid this experience again, choose an idea that:
1) Has a broad-content SEO strategy (think StackOverflow and Yelp). New/valuable content gets created every day.
2) Has a channel for "buying" customers with economics that work. Plenty of people make adwords work. Plenty of others can afford salespeople. Find a market where people are succeeding at buying customers and compete in it.
3) Has a viral loop. Think Farmville or Groupon. Why is it in your customers best interest to evangelize your product? They can be motivated by psychology or $.
4) Create a "tribe". Read up on Seth Godin. Look at Joel Spolsky, 37Signals, etc. They sell good (NOT great) products because they've accumulated followers and evangelists.
It's not about a "great idea". Well, it can be. But look at all of the shitty products that are minting money! You can aim for a "addictive/amazing" product (and should), but it better be backed by sound customer acquisition economics for the (likely) case that your product is merely good.
About your launch day: You expect to blow it out on launch day? That's not a reasonable expectation. It's a marathon not a 100 yard dash.
edit: TechCrunch should not be a goal. At YC there's a word for the period after TechCrunch coverage... "The trough of despair". It's the period of time after TC where your traffic flatlines and you realize that TC isn't a springboard to anything-- it's just the first step (if you're lucky) on a really long slog to building a business.
Maybe his idea, with a 10 degree pivot, could be a viral/SEO sensation. I've no idea.
But certainly some ideas lend themselves better to cheap/free customer acquisition.
GREAT snippet from Patio11's blog (paraphrasing Seth Godin):
"Specifically, he says that the software business is undergoing radical changes because technical competence is no longer a scarce commodity: with open source tools, an increasing supply of trained programmers, and the explosion of cost-lowering innovations for creating and marketing software on the Internet, things which were previously the purview of a technical elite are now within what talented teenagers can cook up from their kitchen table."
A good web app is really only the "entrance fee" to the more challenging (and longer term) game of building a business.
5) Makes sense for other companies to promote to their customers. Ideally ones with lots of customers.
Bonus points: your product fits in to their customer acquisition process (i.e. helps generate more revenue for them).
Triple bonus points: you have access to a large number of these companies through a small number of entry points. For instance a trade association or a friend that will introduce you.
You wouldn't believe how easy it is to get your product out there in the above situation. PR is the last thing you worry about.