Question: First I need to programmatically get tags (unlike what is done here on stackoverflow) from the titles and descriptions of my posts. I don't want commonly used words to appear in keywords. Is there an efficient way of doing this? 

After getting good tags, I would like to save them in MySQL DB.

Now, how do I **efficiently** get related posts using these automatically created tags? E.g. What is done here.

Answer: 
Look up [tf-idf](http://en.wikipedia.org/wiki/Tf%E2%80%93idf). You're looking for terms with a high tf-idf score.
I'm guessing an online API service might help. Check:

OpenCalais - try pasting an article here: <http://viewer.opencalais.com/>

Or Yahoo's Term Extraction API: <http://developer.yahoo.com/search/content/V1/termExtraction.html>

Hope this help!