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: 
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!
Headup may also help with this task (just as Open Calais). You can check it out here, and see if it works for you:

<http://labs.headup.com/Services/RealTime/API/EntitiyExtraction/Playground.aspx>

If needed, there's also an API.

(Disclaimer: I'm a developer @SemantiNet and work on this product... We'd love to help).

Good luck!