We did not allow members to post any self-promotional links at all, but members were allowed to add a link to their forum signature. Often this spurred them on to make more posts, in order to get more eyes on said signature. But there’s a fine line between sharing helpful information, and just posting for postings sake. However, it was seen as a benefit for long term members.
New members were not allowed to add anything to their signature until they had been a member for a certain amount of time, and had made a certain amount of posts (and by posts I don’t mean one word posts or “I agree posts.”)
Alternatively, we also offered a “Connections” section where long term members could ask for, and share, information about their business/products/services, as well as tips and ideas. Members had to have had a certain amount of posts in order to utilitise this forum.
There’s something to be said for offering different level of members, different privileges, it can help reward those who contribute constructively to the community. EG a long-term member can have different needs to a new member.