These posts have been very helpful. Thanks.
At the moment, I can't really add backlinks to increase my backlink diversity, so it is a matter of working on the ones that I have.
I think that we have established that using a keyword phrase (e.g. build metal widgets) that the site is trying to rank for as anchor text may not be a very good idea. The options then are not to have a link in the forum signature, use the site name as the anchor text or to use the domain name (e.g. http://www.buildmetalwidgets.tld). For a site I am working on, build metal widgets and metal widgets are phrases that the site would try to rank for, so I am not sure if having the domain name (e.g. http://www.buildmetalwidgets.tld) in my signature would be over optimization?
[edited by: Robert_Charlton at 5:42 am (utc) on May 9, 2012]
[edit reason] disabled forum auto-link to delink example domains [/edit]