Software Changes
There are a couple of changes you should make to the php software files.
Firstly, backup and open the following file...
Sources/classes/bbcode/class_bbcode_core.php
... find...
... replace with...
... Save and upload.
Basically, this adds a nofollow tag to all links in forum posts, unless they are to your own domain, in which case, they are treated normally. Remember to replace example.com with your own domain. If you rebuild post content from your ACP, it should be added to all old posts also.
The 'nofollow' tag is widely misunderstood. In a nutshell, it tells search engine bots that you did not have editorial influence over this link being added to your website and it will not pass Pagerank to that link. The link is still followed by the bots but you have some protection from spammers etc.
Secondly, remove the 'Powered by Invision Power Board' from the titles. Having it on every page is very bad for your search engine standing. Note that removing this DOES NOT violate the copyright terms of IPB...
Backup and open...
sources/action_public/boards.php
... find...
... replace with...
We also consider it to be a bad idea to have your board name in every topic page title, as it dilutes the actual topic title. You can remove this by opening sources/action_public/topics.php
... find...
... replace with...
Produce the Correct Headers
IPB does not produce the correct server response for error pages. This is surprisingly easy to fix. Open ipsclass.php and find...
Directly after this, add...
This tells the search engines that old topics have gone and the error page will not be indexed. If you wish, you can use the '404 Not Found' response instead, it's up to you. Make sure you check your headers are working properly after this edit.