Mainly, they need to be protected from temperatures beyond their normal life cycles, and have a sufficient supply of oxygen.
New methods of supplying oxygen to bait wells include the KeepAlive brand of oxygen InFusors for live wells, and some similar product available that provide much smaller bubbles in the areator stream.
Here's what Keep Alive says:
"Air is infused with the water at the pump impeller.
The micro-fine bubbles produced are sent gently out of the pump and into the livewell.
KeepAlive Infusors are specifically designed for delicate shad, greenbacks and white bait. The
millions of micro-fine bubbles are so small that they remain suspended in the water longer and therefore provide more oxygen for your bait and catch. Our smallest model will aerate from a
5-gallon bucket up to a 55-gallon drum or livewell, gently and efficiently. KeepAlive Infusors will virtually disappear in a mass of micro-fine bubbles when used in salt water.
KeepAlive Infusors are available as a bottom infusor with suction cups, a floating infusor, or thru-hull infusor. Conversion kits are available to convert your Rule bilge pump or Rule livewell pump to the exciting KeepAlive technology!"
Look for this product to be tested in a future product review.
Aside from oxygen content, temperature extremes are deadly for live bait. You might consider dropping a few cubes of ice in the water at a time to maintain a lower temperature, if you are fishing fresh water. (not a whole bag, Gilligan.)
Nightcrawlers will live in a refridgerator for a long time days to weeks, but not long out in extreme temperatures. Keep 'em cool to cold.