Put fewer fish in each bucket.
Aerate the buckets of water before you add the fish. Keep the aeration going until the fish have been put in their new tanks. You can use battery operated air pumps and plastic multi-coloured airstones. You can buy power inverters (150 watt) from automotive shops and plug them into the cigarette lighter in the car. Then plug a normal air pump into the power inverter.
Add 1 heaped tablespoon of salt for evert 20 litres of water. It helps fish travel better and less die on the way home.
Keep the buckets of fish and water in the shade so they don't heat up and cook the fish. If there is no shade, put the buckets in the water to help keep them cooler.