Give finite invitations to your YCombinator classes and alumni. Have them pass out invites to people they know. Give out more invites when you think you need them. At least this reroots the site back in the "Startup News" seed.
I note that the /old experiment (a Hacker News whose voting data set was limited entirely to accounts > 1 year old) demonstrated no obvious improvement in quality.
It seems to be entirely a question of the public choice of what is acceptable content to submit.