No one knows the secret. If you go to digg front page you can see that some of the articles are with less than 100 diggs, while there are articles in the upcoming page with over 200 diggs. I think it has to do with how many diggs you get and how fast you get them. You can get 400 diggs but if it takes you 37 hours to get that many diggs you wont end up on the front page. However, if you get 80-100 diggs with a few comments, in less than an hour you will probably end up on the front page.