Word of mouth.
I think thats always the biggest.
If you have a good reputation on forums (dont mean post counts, an actual reputation as a member and host) then forums could be a great help.
Even though I dont think its the best thing to do, a lot of people would choose a host if they got an offer on a forum and the host had a large number of posts...Even though that says nothing at all about the company or if that member is just spamming...
Ah well, nothing you can do
If you know someone who has a network of sites they can end up sending a lot of people your way that will signup