One of the best ways to get great links to your site, either it's a Blog, a Forum an e-commerce site or what ever it is, is to never spam a Forum, you don't want to be labeled as a Spammer by a forum Owner, moderator or Administrator.
The number one rule is to never post links in your posts, if you want to promote your site, always use your signature. No matter how good your post is, the moderator or administrator will not look at it the same if it is full of links.
Try to always follow the discussion with relevant posts and make sure you do not use abbreviations while posting. Remember, you are making a post not sending a text message. Divide your Post to small paragraphs to make it easier for the reader to go through it.
Skip the line between paragraphs and do not use a bunch of smilies, one or two will do.
Another great way to get your name known within a Forum community is to contribute with news related to the forums' topic, and never hijack someone else thread and diverse the discussion to something else.
Try also to make as many friends as possible. A member that has a lot of friends within a community, is usually a member that is trusted by the same community. Get your name or username known, and always use the same username.
If you post in many different Forums, always have respect for the rules and regulations. If you feel that the forum need to fix a problem, try to contact the moderator, administrator or Forum Owner directly.
Do not make fun of the forums by bringing things that may put the forum down. For example if you don't like how the Logo looks like, it's better to say: "I think the logo would look much better if you use Bold letter", other than: "The logo looks very bad. It will look better with Bold letters".
These are just a few tips about how to build good backlinks using forums.