We used journo-tech communities like Hacks/Hackers, college boards, company mailing lists (ie. the post design group), or directly in touch with influencers (ie. NYTimes technical evangelist).

I ask because I feel like I keep up pretty well with tech news, reading multiple news aggregators every day - and yet I am clearly not connected to whatever information channels people are using to promote hackathons, since I never hear about them. I have no idea whether I'd want to participate in them even if I did hear about them, but it makes me aware that there must be some avenues for tech news communication that I am totally missing out on.