You shouldn't even try, they'll just ban your domain. It takes teams of people to spam these sites, but if they catch you they'll ban sites, individual accounts, and maybe even block IP's. If your site content is so poor that it's obviously spam, they will catch you.
Are you asking how to spam these sites??? Fist I would like to say that that's nearly impossible but you could always try and tell us how it went.
They are already cottoning onto people doing digg and stumble exchanges and shutting those accounts down, so i wouldnt even bother trying too mate. if you have unique QUALITY content on your site, you wont need to do any forms of spamming.