There are a lot of rules and recommendations that can be given on the subject, but here are the most important:
Avoid Long Domain names - Although the limitation of characters for Domain Names is 63, it does not mean that you have to register the longest possible. In fact, registering short but memorable domain names is always better thanregisteringlongdomainnames.com
Duplicated letters - In most of the cases, a domain name consists of two, three, or four words. Making sure that there will be no duplicate letters between the words will make your domain unmistakable when being typed. An example for bad domain is whyyourregistereddomainn.com
Dashes, dashes all the way - Avoid using dashes for separati