First, harvest when it’s cool. Early in the morning before the sun hits your garden is best. If you have a lot of harvesting to do, pick the most sensitive crops (lettuce and other greens) first, then fruits (tomatoes, zucchini, etc), then roots (radishes, potatoes, etc).
Second, if you can’t harvest when crops are still cool you will need to prevent additional field heat from accumulating after harvest and quickly remove the field heat that is already there.
You can prevent field heat from building up by keeping harvested crops out of the sun. If there’s a shady area close by, move your crop there right away as you continue to harvest other crops.
If there’s no shade or it’s taking up too much time to go back and forth, you can make a harvest tote to keep your crop cool. A simple version can be made by using a plastic storage bin, drilling 4-6 1/2″ diameter holes around the sides, and draping a wet sheet over the top. Draw back the sheet to put your harvest in and then re-cover as you turn back to harvest more. For prolonged harvesting you’ll want to keep a spray bottle or watering can on hand to re-wet the sheet.
Finally, when you get back to the house, garage, or other area where you can work in the shade, you will need to remove the residual field heat by cooling the crop in water. Leaf crops can be dunked in a tote of cool water, swished around for about 30 seconds and then set out to dry in a place where the water can drain off and they will not heat back up. A wire mesh screen set up over a sink or on stands above a garage drain works well.
With your harvest now cool and dry, it’s ready for storage in the fridge and you can expect it to be crisp and fresh for up to 7-10 days.