Two ways 1) Use some keywords in it like search.com. They have tremendous advantages in search engines. 2) Use a brandable domain like google.com. In Web 2.0, many companies are preferring brandable domains. 2 will be the obvious choice for many as all keyword, keywordkeyword and keyword-keyword domains are taken.
One rule I have now is to never, ever buy on a 'Wow...that's cool... and available!" type of whim. When I've done that, I've invariably regretted it a short time later...