Alan,
When the bait arives at the shop it's usually in sawdust and sweating already, to prepair my bait this is the steps I take:
Use a shallow tray for these steps,
1) Riddle the bait removing the sawdust.
2) Put a handfull of maize meal in and rub the bait through it removing the sweat and any remaining saw dust and riddle again.
3) Put the maggots on the riddle a small amount at a time and throw away the dead ones and skins left on the riddle.
4) Place the bait into a large tub (I use a 3 pint box for each pint of bait) and put a handfull of maize meal in each box.
5) Put the boxes in the fridge turned up high without the lids on for about an hour, then put the lids on (if you put the lids on straight away the box will have condensation produced by the bait which makes the maze meal stick to the bait and this can kill them or make them float).
6) The night before going I put a table spoon of powdered flavour in after riddling them again and some fresh maize meal.
7) When at the venue keep the bait in a coolbag only removing enough to get started and top up as you need this keeps it as fresh as possible and you can put it back in the fridge if you don't use it all.
Hope this helps.