Search feature is one of the most important features on a website because a study revealed that 83% of users use it regularly. The search control should be easy to find and use.
Search input field should be visible, kept simple and wide enough for at least 30 letter characters.
A product or service that users can't find doesn't sell or exist in their perception thus making it a critical element of building a profitable website.
Search should be accessible directly from the homepage as most users expect an input field and a button next to it. If they don't see one, they assume the website doesn't have a search feature.
The search area doesn't require a special 'Search' label. Instead, labeling a button 'Search' is the simplest way to explain where the search is.
Search should be placed at the website's top, but below the banner area. This way search has the greatest chance of being found by users when they need it.
Options for advanced search should be a separate link below the search area if it's used frequently. The exception to this is when the advanced search is the website's norm and no simple search option exists.
If the advanced search is used infrequently, it shouldn't be included as a link on the homepage. It should instead be presented after the user gets search results.
Search on the homepage should search the entire site by default. Users will assume that, so it should be the default behavior.
Offering 'search the web' feature next to the website's own search will make the the search process more complex and error prone. Users also prefer using their favorite search engine on their own if they want to search the entire web.
Logs of what users look for using the website's search can give answers on what they are looking for, how they think and where the content could be optimized to meet the user's needs.
When a search fails to produce any results, a good practice is to lead the user to use the site's navigation to find what he is looking for.
Users often make spelling mistakes which can severely influence the search results. If no results are found, immediately present the autocorrected results.
Relevant but obsolete results aren't as interesting as relevant but fresh results. Try to sort search engine's results by relevance first and then by date as well so the users get the best of both worlds.