Standard and Europallets:
Knock off the underside tie strips.
Select 2 house bricks and support a slat. Drive down the bearer, hence drawing nails. Move bricks to next slat. Repeat.
This should remove all slats , with nails left strait for hammering , or driving back.
Remember all blue (CHEP) pallets are not legally retained or repurposed.
All wooden pallets imported (or exported) will have been treated with insecticide.
The lifespan of salvaged slats can be extended by treatment with raw linseed oil. Cheapest source for this is from horse food suppliers (such as Mole Valley Farmers/SCATS), where it is VAT Zero rated.