Site Owners Forums - Webmaster Forums - measures for not losing ranks?

Those who are making money online, losing ranks can adversely affect their earnings. Still worse, for those who are abjectly dependent on websites for their livelihoods, losing all search engine traffic can be simply disastrous. When sites lose traffic and drop down in the rankings, the one thing everyone immediately thinks of is - a penalty has been applied to their sites. Penalties are certainly applied by search engines but only when you have resorted to some undesirable activity like using cloaking, hidden text, doorway pages, keyword stuffing or link farms etc. So what measures can be taken to prevet this from happening?
Any help is appreciated!