Link building is time consuming and can often feel unrewarding for the effort that is put in. It is still an important way to improve traffic to your forum and to help with the link popularity that is so important in a web site’s ranking in the search engines (ie building backlinks). There are many ways to get links, but the time needed to do this has to be weighed against spending the time making the forum an awesome resource on a topic. The better the forum is as a resource on the topic the more likely are other sites to link to you. It is these editorially given links that are much more valuable than the self promotional links that you can get by submitting to directories and social bookmarking sites. Personally, I use a service to submit sites to link directories and social bookmarking sites. For the amount of money involved and the time it takes, I am better off spending time on the site to make it better, so I can editorially earn the better links. Links from directories and social bookmarking sites are not high value, but they do have their uses. I recommend Directory Maximiser and use them to submit sites to directories for 14 cents a directory and Social Maximiser to submit forum threads to social bookmarking threads for 19 cents each. When I have a forum thread that is a valuable resource on the topic of the thread, it is time to get some links and traffic to it. I work at improving the quality of the content of the thread, while Social Maximiser do the work for me. I have been happy with their service and the results. They submit to just over a 1000 directories and 130 social bookmarking sites. The cost of this is offset with income from the site.
If you have the time to spare and do not want to use a paid submission service, you can do it manually yourself. You may find the use of submission software that partially automates the process to be helpful. I previously used Submitease before using the above service and was happy with the software. It made submission and submission tracking easy. They regularly updated the lists of directories available to submit to. It does cost $78, but there is a free limited trial, so I recommend you try it.
There are also some paid link directories that are probably worth submitting to if the budget allows.
Just how much are these types of links worth? No one is sure and everyone has an opinion (and I have mine). It could be, as they are easy to gain, that the search engines filter them out or no not value them with much link juice. My opinion is that they are worth it in the early stages of a site when there are not many backlinks to the site. They do not seem to be worth as much when there are already a lot of backlinks to a site. Also, keep in mind that your competitors are probably doing it.