The real answer you don't want to hear: do a product that people want, the rest will follow.
I wrote a blog post a while back about what I called the elevator effect, which basically gives an example of riding a wave that is going your way. As you get a first burst of publicity, you can channel that to get more publicity and more exposure. If you ride the wave well, you can go pretty high.
In your question, you ask about Digg. Do you ask about digg because that's all you have heard of, of because that's where your target audience is?
Having been on the receiving side of high bursts of traffic, I can tell you that not all bursts are productive. Sometimes, these visitors would never be users, so although the raw numbers look impressive, it's not real.
So I'd recommend to first figure out where the people you want to reach are. Then focus on how to reach them.
For instance, for a certain startup I won't name, they had the opportunity to be featured on TechCrunch, although it wasn't their target audience. It was useful because that coverage helped them land coverage in more specialized magazines, where their audience was.
If you play pool, you know that straightshooting is not always the way to go :-)