Some internet marketing techniques can do more harm than good.
Forum signature links are one of the easier ways to get links, but are considered spammy. Although it's good to be diverse in link building strategies used, this is one to use sparingly if at all. The way these work, is you create a forum profile and in your signature, you make a link. Then you go into the forum and post some comments. All the comments you leave will show your signature. You might even be allowed to enter hand coded links right into the forum posting itself in addition to your signature link. If your sole purpose is to build links as you comment, you are doing nothing more than contributing spam.
A similar link building method pertains to leaving comments in blogs. Some blogs require you to create a profile before you can post. Other blogs might allow links in the comment itself. Most blogs are mandated by an administrator who needs to authorize your comment before it gets posted. For this reason, blog commenting can tend to not be quite as spammy, but again, if your sole purpose is to build links as you comment in blogs, you are spamming the blog.
Before I throw this strategy under the bus, I must qualify what a mean by a link exchange. It is okay to exchange links with friends, colleagues or business associates that want to help each other by providing a good outbound link resource that has something of value to your website visitor. What is not okay, is when this strategy is abused and people think more is better.
Do you ever get those emails that say, "I came across your website and noticed that, (blah blah blah) and wondered if you want to exchange links". Before you become to flattered, you must realize those are not personal emails from people, but rather generated from software that automatically seeks out potential linking partners. If you get emails like this, 99% of the time it is a website you want to stay clear from.
Most of those sites seeking out link partners already have tons of links on their site and are constantly growing more and they hold zero value to you. They might benefit from you (because you have less links), but you gain absolutely nothing from them and if fact it could reflect negatively on your site. So just delete those emails.
Most of what is mentioned above is reciprocal linking, or in other words both parties are linking directly to each other. As mentioned earlier reciprocal linking can be okay if not abused, but one-way links are better. One-way means someone links to you, but you do not link back to them. For instance, if you can get a link on your local chamber of commerce without them requiring you link back, that is a one-way link coming into your site.
With that in mind, some search engine marketers came up with the three and four way linking:
This is a black-hat link exchange that people use to try and trick search engines into thinking they have unique one way links when really it is a scheme to manipulate one-way links. Please stay clear from these link exchanges and only use white-hat seo techniques.
Search engines are smart enough to pick up on these spam techniques and do not like them.