According to my experiences, the best is if you use at max 15 of the most relevant keywords as tags. More makes more harm than good, so try not to use more than 15 words. (I don't know how long your articles are usually, this information was deducted from my hubs, which are usually 900-1000 words long).
Additionally, it is VERY important, to use each keyword/tag AT LEAST 3-5 times in your article, as this helps you to get better places in search results. Google relies a lot on these keywords, so I guess this is the most influential thing.
The other that can help you in achieving more hits is the good summary - but this is
another story :).