I believe the less time a fish stays in the livewell from when he was caught vs. when he is weighted is the most important part. For that reason I try and catch and cull as many fish as possible. :)

On a serious note, Sacto mentioned the running freshwater through the livewell and this is huge. I run my pump based on size of fish in the well and # of fish in the well. meaning if I have only 1 small fish I dont run it that often I just add water every 30 minutes or so to give it oxygen. If I have anything over 3 I will pretty much run it all day. I also use fish dope when the air temps start to rise until late fall and allways on anything over 6lbs.