Here are some other factors I have noticed in improving quality score:
Having the "main keyword" in your url
Let's say you have a ringtone site and you have lots of keywords of this nature: eminem ringtones, 50 cent ringtones, akon ringtones. From my experience, you can raise your quality score if you have the word "ringtone" in your url. So, the following urls would be good: SuperRingtones . com, RingtoneCentral . com, etc.
Having a high CTR will increase your quality score
From my experience, any modifications to increase your quality score in your landing page will at most get you a "good" quality score, however your bids will still range from .10 to .40 cents. In order you get those magic 2 cent bids, you will need the keyword to first get clicks. If you are fortunate enough to get a high CTR, your quality score will become "great" and the minimum bid will adjust accordingly.
There are just some keywords you can't get a high quality score for
As much as I try, there are some keywords I can't maintain a good or great quality score. The main reason is because the keywords are not super targeted. I think some keywords I tried are "akon", "gwen stefani". I can profit if the bids are like .20, however even though I have a decent CTR, I can't get below .50 minimum bid.
PM me if you have any questions on this.