place bunches in plastic bags, and keep them out of the sun so they don't cook (greenhouse effect). Leave the end of the bag open just a little so excess humidity can escape...but there is no perfect way...maybe mist occasionally at market.
Ann

Put one package out while you keep the rest in the cooler. When it looks like it's receiving too much sun/heat, switch them.

I try to pick when it's cool and dry and don't wash the leaves. I cut off the stems and put them with the leaves into a 16" plastic bag, like you see on rolls in the supermarket, and try to keep the ends of the leaves a few inches below the lip of the bag. From the time I pick until leaving for the market they are in the fridg packed upside down in "thank-you" bags so there is no exposure to fridg air. I keep that way in a cooler at the market and take out a couple at a time. Keep in the shade and don't let the leaves be exposed to the wind. Ours looks pretty good all market.

i cut in the early morning, wash the leaves and bunch them with a tie around the stems. they go into a small cooler that is lined with a couple of ice packs covered by a damp towel. i stack them and then place another damp towel over them. the next day at market, i spritz them and put as many as i can in shallow plastic containers with cool water in the bottom. they last well for me.
keep spritzing all day and try to make an eye catching display so they sell fast. that way you won't even have to worry about keeping them looking fresh!
make a nice sign, have some recipes ready and pile the bunches high. good luck with your chard!

Thanks for the excellent advice! I wish I could do morning harvest, but I leave for our Saturday market about 4:15 a.m. Short of installing stadium lights, I'm pretty much out of luck. However, you have shared exactly what I was looking for and I thank you all!
DD