One of the most important element, if not the most important element, in Search Engine Optimization, is link popularity. This refers to how many inbound links a website have--how good its link network is.Several websites promote links by putting them in a directory. Doing this means more links and, hence, more traffic. Even better is trade linking, because it creates a win-win, mutual relation between two websites.
Personally, I haven't tried trade linking before. I'll try it now. I hope you do, too.
Free Link Exchange, Link Swap, and Link Trade Directory