1. Similar/Synchronous to the website name.
Select the domain name which is either the same as the name of your website or synonymous to it. This helps you to get more clicks. Websites are normally searched by names, thus the visitors will click the link by default.

2. Based on the Business/Brand Name.
Emphasize your domain name as your own brand   name or the brand of the product you deal in. A generalized approach will not help you to make your presence be felt worldwide.
Example: If the website is for a driving school then as the first point suggests it could be smithdrivingschool.com, so the search starts by the name
OR it could be drivingschool.com rather than business.com
If such a domain is found to already exist, choose an option closer to it or check the whois information and try to purchase the domain.

3. Modification in domain names.
In case a domain name of your choice is absolutely unavailable, then the keyword can be modified. For e.g. your desired domain name is blackboard.com, even black-board.com is also already registered.
 Then try search for MyBlackboard.com, TheBlackboard.com, MyBlackboards.com, TheBlackboards.com etc.

4. Use of Keyword in a Domain Name.
 A keyword is nothing but a search term or query we generally type in search engines and search engines delivers the results accordingly. Every webmaster wants his website should index at relatively higher ranking in the search results for a specific keyword closely related with his site or business...