you can take the inside of a metal floating minnow bucket and put it in the water someplace out of the way, tied to the boat. soons you get anchored put your minnows in this bucket, all but a few to fish with. as you run low in the fishing bucket, dip you some more outta the floater. If you got a lot of minnows, this will keep em alive all night.
a lot of bait stores keep their minnows in refrigerated water. much cooler than the lake water, so they will be lively at the sale. once paid for they dont care how long they live. acclimated to cool water and put in warm water or vice versa will do em in. find a bait store that dont refrigerate their water and youll have longer lasting minnows most times.
this time of year its hard to keep minnows alive, no matter what you do so keep a check on em on the hook and change frequently as youve already found out, they dont live long on a hook when the waters this warm. try hookin em up through the lips instead of through the eyes or back.