No idea about your location as I am in the NE but usually you look for muddy rather than sandy ground at low tide close to waters edge, just dig a trench. I was told to place them in a tub of clean salt water which if you change it regularly and remove dead worms, they will keep for ages.
Only ever dug for them myself once and it was in a mussel bed very muddy sludge. Only time I dug in clean sand I only found the odd small white one.