Keep your domain name short, if possible, since short names are easier to remember. However, a long name with more keywords might work better for search engines. Consider carefully before deciding if you want a short, easily-memorable domain name or a long, meaningful and highly accurate one.
Keep your domain name simple. It should be easy to remember, spell, and say—no tongue twisters! This ease of spelling should produce difficulty of misspelling. (You can buy misspellings of your domain name, too, to help matters.)
It should also be dissimilar enough from other domain names to prevent confusion, which adding, for example, "the" to an already-registered domain name would produce.
Beware of trademark infringement! If it is available, you can buy a trademark domain name only if you use it consistently! If you build up your site continuously under that domain name, a court case will probably uphold you since there is precedent. There is also precedent of a company buying trademark domain names solely to resell them to the companies. That company was forced to relinquish all those trademark domain names.
Beware of your domain name's expiration! It has happened on a number of occasions that, for example, an author registered a domain name, had it printed in his book and publicized—and the domain name expired, without his knowledge, for his registration company did not tell him. Within a few days, his domain name was bought by someone who turned it into a porn site. Do not expect your registration company to tell you when your domain name is expiring! Keep track and keep payments yourself. Because of this, it may be better to go by the less convenient method of personally renewing your domain name each year, so you have it as a habit.