This kind of hack is not programmatic or via some robot of any kind.  It's an editorial hack.  I learned it through trial and error, though I must admit that I didn't fully test it.  Here's how it works:

1. You need to start reading Slashdot carefully and get a feel for what kinds of posts make it big.  There's no formula, but there are certainly some patterns: 
   • It must be a breaking story.  By the time it reaches a printed newspaper, it's probably too late.
   • News isn't enough.  The post must be insightful.  They don't care about product releases unless it's something that will change a market. 
   • The post needs to take a position.  It must imply a threat to a traditional technology or offer a key evolution of a technology.
   • Language is important.  The moderators look for bad writing and may kill a post based on it being poorly written.  They may even change the headline.
   • Excerpts, quotes and pointers to the source are required.  Sometimes a post will read like a footnote from an academic journal.

2. You need to start posting with some frequency.  Your first post won't likely make it.  The moderators are so close to the system that they know certain posters by their handle.  A new guy needs to prove himself and write several entries before being accepted into the club.

3. Reference several sources.  They may pick up on the fact that you're trying to drive traffic to your site, so link to your competitors now and again to disguise the scent of bad behavior.

4. Help them moderate.  After your first or second accepted post, you may be invited to be a meta moderator.  Take it seriously.  Help them moderate.  This will boost your karma.  And the higher your karma, the more freedom and power you earn.

This hack worked for me briefly back when I was at InfoWorld.  It's certainly not a guarantee, and even when a post is accepted that doesn't mean it will be a winner.  I found this effort tiring, as I had to really think about each post while I gamed the system which took too much time given the demands of my real job.  But I think I had a pretty good hit rate of around 20% after I figured out what they wanted.  And when you get one that makes it really big, then the effort suddenly seems trivial.

I wanted to hire someone to do this for us, as it would have been a much better use of traffic marketing budget, but you need someone who knows your content really well.  You're only going to find that kind of talent on your editorial staff.  And then you get into some weird integrity issues when your editors are also flogging stories.

Your on-staff marketing people, however, should be smart enough to write an intelligent Slashdot post.  Get your editors to train them.  Make them do some real work for once