Back in the BBS days getting people to call your BBS was hard work. Depending on the intended audience (general BBS users, complete underground or anything in between) you had to get people to notice your BBS and check it out.
Notices on other BBSes Edit
A lot of BBSes had listings of 'friend' BBSes, for example in the logout screen. A Sysop could ask the Sysop of a nearby BBS with a similar audience to list your BBS.
BBS listings Edit
Several BBS listings were maintained with simple sign-up forms and room for descriptive text.
Mentions in software Edit
Documentation for software listing the 'distribution sites' for that software listed BBSes which had the latest versions for download.
Mentions in GIF images Edit
GIF images for download with the name/number/other info of the BBS included were a way some BBSes which were mainly active in graphics advertised. A sample collection can be found at http://www.flickr.com/photos/chinesejetpilot/sets/72057594071693402/
Advertisements in print Edit
The way to reach the beginning BBS user.
Classified advertisements Edit
The small advertisements in the back of magazines/newspapers were not free, but for a hobby BBS an affordable way to reach new users.
Bigger advertisements Edit
Usually limited to commercial BBSes: bigger ads (quarter page or more) in colour.