QS is a comparison of your click rate to that of others. If you want to increase QS, increase your click rate with better ads.
I believe quality score has more to do with particular keywords than ads, unless you are talking display network instead of search. For keywords it has to do with how relevant the keyword is to your site, if Google considers your site to be "worthy" overall and how fast your site opens. Anyway, here's more on this subject: https://adwords.google.com/support/aw/bin/answer.py?hl=en&answer=140351
I'm talking about search of course. The QS is shown at the keyword level (why Google does this, I don't know) but your ad is the driving force behind if it is clicked or not. Sure, the keyword will have an effect if you are silly enough to use "color printer" when you are selling monitors, yet, many advertisers do this. So yes, keywords have to be relevant. It represents about a quarter of the QS. How fast your site opens represents at most 5%. Most of QS is CTR.