I've actually tried this. First of all, your tweet needs to be completely authentic. Second of all, it's not only retweets, it's favorites as well. My personal opinion is, it won't go viral unless your account is over a certain age. I think this has something to do with the ability to link spam if you could just make a few thousand accounts and retweet/favorite a tweet on them all that involved the trending hashtag.
Also, the accounts that retweet/favorite the potential viral tweet have to be established as well, because I've personally tried the trick that I mentioned to force the tweet to the top.... and it only worked once, even with hundreds of retweets and favorites.
I think also that the top 10 trending topics are monitored, and someone that works for twitter can delete your tweet off of the top tweets if he deems it not worthy.. Whether that be copyright, faked retweets/favorites... or anything else.. Just to keep the list from being a spam fest.
Of course all of this is speculation, but from what I've tried, it doesn't seem possible to make it go viral without having a lot of tools and aged accounts to help.
The time I succeeded was really nice, but I'm not sure what made the difference.
