Do it consistently
Visitors learn for example how a website marks the links or where the "related-content box" can be found. Consistently adhering to these established "codes" will make it easy to navigate and read the site.
Breaking these conventions will interrupt the flow. Such distractions might be enough for the user to leave. When creating a site it is important to define all the conventions and rules that are used. Consistently following them is essential for giving the user an easy time when visiting.
Don't mislead the visitor
Although links can look however the screen-designer decides, certain standards have developed and find widespread use. Underlining a text for example is a common indicator.
It is good practice not to underline text, marking it blue or putting an arrow in front, if is not a link. Visitors might construe them to be clickable.
The same applies to images. Many users will try to click on a graphic or any other image. Very rarely, they will find an active link - a tiny, but nevertheless negative experience. Captions have proven to be effective to add that little bit of content that users need in order not to click on an image. And if a link is present, it can be placed in the caption itself.
Show used links
Marking which links have been visited is very valuable. It helps to quickly "tick off" when going through a site or helps in finding this piece of information from a previous visit. Unfortunately, a great many sites do not use this very basic feature.
If the "visited" feature should also extend to the navigation is debatable. On very deep sites it might be useful. On the other hand it might confuse the visitor. The navigation should be a constant and change only minimally.
Match the destination with the link
Clicking on a link can be compared to following road signs in a city never visited before - you are very happy if you get confirmation that the choice you just made was the one intended.
Same thing applies to links - if a link reads "find out more about our services" the page that it relates to should show the words "Our Services" somewhere prominently in the title. It confirms the action taken.
Well written titles indicate clearly what the main topic is on the page. This is very useful, especially considering that a lot of traffic directly dives deep into the page.
Embedded links used to their full advantage
Links are visually different than the surrounding text. Color, a markup or other cues denote a link. Something maybe even happens before you click - mouse-over effects or an overlay, displaying the title.
Everything is designed to draw your eyes away from the surrounding text. If you embed links within a sentence, readers will very likely notice the link first and only later read your content. On first thought that is unfortunate, but it can be turned into an advantage.
Since reading online involves lots of scanning, the eye is always looking for visual cues. They can be provided by using subtitles or in this case descriptive links. By creating a meaningful link, finding the relevant information on a page is made easier. "Click here" is less effective then "View a list of articles about usability".
How to link to files
When linking to a file it is likely that another application is opened and that the visitor is drawn away from the site. Not something that should be done lightly.
A great many times websites link to related documents, very often PDF files. What exactly can be found in the file, the visitor needs to figure out from the link itself. The first step in creating a good link to a document is to summarize the content. That gives the visitor information how he wants to proceed.
The second step is more basic by asking why the document cannot be created as a webpage. If it is worth to be put onto the site, it surely is worth the effort to make if searchable, fully integrated and less work for the visitor. Using for instance PDF-files can make sense. Very often however, it is simply easier for the developer to put it onto the site - but not easier for the visitor to use it.
Mark the external links
An internal link refers to a page or file within the current domain, an external link usually points to a site, housed on a remote domain.
Although technically there is no difference in how the code for the links is written, it is common usage, even a w3.org recommendation, to inform the user if an external site is accessed by a link.
The rationale behind this is not to confuse the visitor who is getting used to the layout, navigation and architecture of a particular site. An external link can either be declared by stating it in the text, by a specific icon (like for instance Wikipedia does) or even by using the "title" tag of a link.
Whether the new site is opened in a new browser-window or replaces the current content could depend on the kind of information the visitor will find. It could be argued that sites with related content or sites who delve further into a topic might best be opened in a new window. The visitor can more easily relate back to the original site. In that case not only the external link, but maybe also the fact of a new window opening should be placed with the link.
Creating good links is vital for creating a good site. A link for instance is the only possibility an e-shop has to have users put products into the shopping basket. If that link should be badly written or confusingly placed, many users would give up before buying. That would be like not accepting money from a client standing right in front of you.
