Link rot is a significant danger to Wikipedia because of the reliability policy and source citation guideline.
In general, do not delete cited information solely because the URL to the source does not work any longer.
Links added by editors to the English Wikipedia mainspace are automatically saved to the Wayback Machine within about 24 hours (though in practice not every link is getting saved for reasons ranging from quirks of a website's code to robots.txt preventing a website from being archived).
This is done with a program called "NoMore404" which Internet Archive runs and maintains; other language wiki sites are included.
It monitors EventStreams API, extracts new external URLs and adds a snapshot to the Wayback.
This was incomplete but a significant number of links were added to archive.today during this period making it a major archival source filling in gaps of coverage.
As of 2015, there is a Wikipedia bot and tool called WP:IABOT that automates fixing link rot.
It is a flexible tool that can carry out many custom jobs such as URL migration/move, usurped domains, soft-404 discovery and repair.
Suggestions for ways to manually improve archiving: Most citation templates have a |quote= parameter that can be used to store text quotes of the source material.
To indicate that all used external links in an article have been successfully archived by the date of edit, you can add the template {{Archived reflist}} at the top of the article's talk page, which renders as: When a domain on the Internet expires, anyone is allowed to pay for and control that domain.
Add-ons (extensions) are available for most browsers to search for archived copies, with names such as Resurrect pages.
Usually dates closer to the time the link was placed in the Wikipedia page, or earlier, are more likely to show valid information.
In that event, consider finding an alternative source so that the loss of the original does not harm the verifiability of the article.
Sometimes, finding an appropriate source is not possible, or would require more extensive research techniques, such as a visit to a library or the use of a subscription-based database.
Permanently inaccessible convenience links for print sources can be removed, but the reference should be retained.
With a dead link, it is possible to determine if it has been cited elsewhere, or to contact the person originally responsible for the source.
In truth, there is not a lot that can be done; maintenance of non-Wikimedia websites is not within the scope of being a Wikimedian, nor in most cases within our capability (although if they can be fixed, it would be helpful to do so).
However, it may be good practice to think about the potential impact on other sites when deleting or moving Wikipedia pages, especially if no redirect or hatnote will remain.