Here are some guidelines/rules of conduct from forums with varying degrees of similarity to this one:

http://giftedkids.about.com/gi/boards/proxicom/guide.htm
http://www.artofproblemsolving.com/Forum/ucp.php?mode=terms
http://www.bellaonline.com/misc/forums/rules.asp
http://www.bellaonline.com/misc/terms.asp
http://www.bellaonline.com/misc/forums/compassion.asp
http://www.giftedhaven.net/forum/showthread.php?tid=1212
http://www.mothering.com/motheringdotcommunity-user-agreement
http://www.welltrainedmind.com/forums/faq.php

The above sites have the following rules on spam and solicitation:

* No commercial messages or solicitation, but can include links to a personal website in a signature (giftedkids.about.com)

* No advertising, promotional materials or solicitation (artofproblemsolving.com)

* No posting with the main aim of driving readers away from the forum website (bellaonline.com)

* No posting to advertise a product, business, website or blog or in any other manner from which you would financially benefit (mothering.com)

* Authors of homeschooling or other materials, or who have a financial interest in a particular program, may answer questions about those materials/programs but may not use a more general query to promote their materials/programs (welltrainedmind.com)

I don't know if I'd recommend a rule that no one involved with any website can link to it. That would rule out even links to Wikipedia posted by a Wikipedian. I don't necessarily think that a person could never link usefully to their own blog either. I think a pattern of linking to a website where one has an interest could become solicitation.

If we are to allow some links to websites where a person has an interest, I don't know whether it would be optimal to allow links only in sigs. That would mean that sometimes a person might know about a page that could be highly relevant and useful, which happened to be on a website where they have an interest, and not be able to do more than hope that the reader would find it by accident by following the link in their sig. A sigs-only rule has the advantage of being fairly easy to apply, being fair, and avoiding a lot of solicitation, but the drawback of excluding some useful content.

I would generally in the past have considered these factors when deciding whether posts were solicitation or spam:

* Apparent intent to drive traffic to the linked-to website, instead of simply participating in discussions on the forum site.

* Pattern of posting links to a single site. (This feeds into the last factor.)

* Pattern of posting more than one link to the same external website in a single post.

* Posting links to the same external website with low relevance or which are unrelated to the discussion.

* Relatively few/no forum posts without links to the external website, in proportion to posts with links.

* Relatively low amount of content when posting links to the same external site.

* Non-link posts with minimal or negligible content, such as "Okay" or "I agree", in an apparent attempt to counter the number of posts with links or to present a link in a signature.

* Any interest in the external website linked to in the ways specified above, but especially a financial interest.