I spent quite a bit of time looking in to this as my site suddenly dropped out of the listings.
You need to add a robots.txt file to tell google what to look at and what to ignore.
You should also add a sitemap.xml file which will help describe the website for it to ‘crawl’.
Log your website with http://www.google.com/webmasters/ and this will tell you when it has been crawled and how often.
Regularly update your website with new info/articles.
Get other sites to link to yours (be careful with this one) this can get you considered spam if you muck it up.
Avoid sites which pay for adding your site to search listings. Also worth manually submitting to yahoo and google.
If you don’t understand the terms above then do a search on google. I had no idea what they were all about but did plenty of research.