There are many hosting sites available this day. Unless you have unique selling feature, you may have to consider ppc marketing. Many hosting site offer free domain name when user signup for their hosting plan.

There are number of hosting companies out there who offer free domain name for a year when you sign up with them.
Some Promotion methods of web hosting business :
I think there are lots of good forums where web hosting discussed You can join this kind of forums and contribute and offer help to others members while keeping your signature with your URL at the bottom of your post.

Word of mouth is very good way to promote your web hosting service. To make one of your customers refer other customers you should be providing a very good service to make that customer happy.