Back to the topic, this is useful:
1. Check your stats every day or at least once per week
2. Keep a record of important updates or modifications
3. Check the effect of #2 over #1. Period.
This way you can see if the traffic started going down since you change your metatags, keywords, content, design, tables to css, css to tables, and so.
If you didn't change any of these things, then is a content thing. You probably published something others didn't *like (please see the *point at the end). If you did nothing to your site, then thats another explanation, no update... Also check for the availability of your site (uptime).
* Google provides traffic, remember your links are really important for your clics and your G traffic. A site with no links means a dif thing to Google if you have 5,000 links. So, perhaps some sites linking to you are dead or they just removed the link to your site. Is not the same as if your site has links from newspapers (traffic coming from them and also from G) that if they remove the link to you. Yes G sees that too.
There is a lot of discussion about what G thinks when some sites remove their links to yours.