Search engines place major importance on external sites linking to yours. Your best results for gaining link popularity is to search the internet for relevant high ranking category websites and see if you can get linked within them. Today's search engines are all about popularity and not relevance. Google is a popular Search Engine that only likes and awards popular sites. It is becoming nothing more then a game for links, so much so that there is more spammy links on sites now then ever before and black hat link building principles seem to be far superior to white hat and more natural approaches.
Sites get their ranking based on link popularity first then on their on-page optimization. So your must have links to achieve a PageRank (PR) then you must optimize the site to be above the other sites with the same page rank. Most sites will never achieve a page rank higher than a PR5 or PR6. So you must be well optimized and well linked to beat your competition. You also need to consider the right trust value in the links pointing to you. Authority domains, which now are as guilty as link buying and link spamming asany other site are still over-valued by some Search Engines, as if to say that they do not even trust their own results so they would rather rank a site like Wikipedia or a proce comparison tool with many categories then a site that actually better represents the terms searched. Its sad but relevancy is no longer sought be Search Engines, instead several forms of popularity are becoming more important as they loose the battle in fighting spam.
The best links are not reciprocal links, but links you do not return termed "one-way" links. Incoming links with no return links on sites with similar content are the best quality of links to have. They return the most value. Sites that offer good information, articles, free tools and resources generally have these quality links. This establishes them as an authority site.
Advertising for Links & Content:
Links you pay for by ads on similar resource sites will build both your Page Rank and your Search Engine Rankings and really good links will also drive in traffic.
These are the cream of the crop in today's search engine world. But to cut expenses and be ranked longer without paying, you must find links into you that you do not pay for, and will generally remain online for years to come. This can be the hardest endeavor. You can hire an SEO Company that offers Link Building Services to do this for you. Or find a reason why other webmasters would like to link to you. This can be in the form of a quality web directory on your site, a tools section, a resource center, tutorials on your service, newsletter, information about specific products. The better and more realistic the reason, the better your returns will be.
Search Engines can tell the difference between reciprocal links and "One-Way" links. One way links help you more than reciprocal links. Getting one-way links is harder than getting a reciprocal links. Ideas for one-way links... give them a reason to link to you. Most one-way links are received by websites that use "Link Baiting Ideas". They find a way to get you to link to themselves without returning the link. This can be done through high quality content that people will want to reference, social website interactions, awards, giveaways, free tools or any other method that makes people want to add your link.
What to Avoid:
Avoid linking to websites that have a Google PR 0 (Google Page Rank 0), these numbers can be found in the Google toolbar a plugin for your web browser. These PR 0 websites could be new, in which case they would be safe, or they could have used bad SEO techniques that have cause them to be removed from the Google index in which case they get a penalty in the PageRabk toolbar.
Avoid companies that email you and ask you to link to their network of websites. Especially when its over 5 websites all on the same exact topic. This could hurt you down the road and connect you with these websites. If these webmasters use poor methods of ranking, they you could be penalized by ranking to them.
Avoid bulk email link requests. These usually look like "Hello we have a website that is in the same category as your website and we would like to exchange links with you..." These companies sell links, and they send an email to every email address that they have on a purchased list asking for a link. The quality of this reciprocal link is usually poor and will more often hurt your website. A good link request would have a link already added to your website and some Google PR.
It is just as important to link to other websites
Avoid Link Schemes, Link Farms and Link Popularity Services that require you add code to your website. Some companies will put the same links on every page of a website and tell you that this is a safe method. You now have duplicate content, duplicate links. These will help you for short periods of time but they could hurt your own rankings in the near future.
If these plans were so good, every SEO would use them.
Be cautious of websites the hide their link pages (on other domains) and/or only offer a link to their link pages from their home page and not the rest of the website. If the link to the links page is only off the home page and not part of the footer than the links could lose value. In some cases these link pages can help you if the home page has a high Google PR. A worse case scenario is when they only link from the home page to the main directory and then they have several categories in the directory, the problem with this is that the PR gets watered down the further you go into a website. This is also bad for the website itself. In this case they have created several lower ranked pages in a directory that all link to their home page on all those pages. This adds too many low ranking pages linked to their home page and hurts their own "Internal Link Popularity". So in effect these link directories hurt their owners ranking.
as it is to have links incoming links to your website.
DO NOT HIDE YOUR LINKS -- Flaunt them!
BUILD QUALITY LINKS NOT JUST ANY LINK!