Post on every forum you can find (not spamming them) with your URL in your signature. Go locally, talk to local businesses - join your local chamber of commernce.
Online you could try directories like findmyhosting.com where they send you leads - you have to be good at trying to sell to someone though as other companies get the same leads.
Host your friends and families website and get them to tell friends about you, give them some business cards to give out incase anyone asks them about their website.
Have you ever hosted anyone for free? sometimes that does work depending what it is, if its a fan site then maybe you can get the people who are making their own spin off sites etc.
Do you have any hobbies, use any other types of forum? where people may want to set up their own hobby site that you could host? maybe offer them a discount and then they can tell their friends.
Those ideas are all cheap or free of course there are more expensive ideas, like paying for advertising on sites like WHT but from what i have heard that builds your brand more than gets you customers - Just think how fast those impressions must get eaten up at busy times!
Hope thats of some help, we are trying to do the same ourselves at the moment, as we have just re-designed the site.