Make sure you get nice fresh maggots (check the black food sack is visible under the skin on their back). Riddle out any dead maggots, skins and bits of rubbish.
Clean them by mixing in some (a couple of tablespoons per pint of maggots) maize flour (sold in supermarkets for making tortillas and polenta) and giving them a good shake. Leave them to wriggle for a while and clean themselves off, then riddle off all the flour.
Put the maggots in a suitable size freezer bag, squeeze as much air out as possible without squishing the maggots, and tie a tight knot in the top (you can use a straw to suck the air out if you are brave). Put them in the fridge. After a few hours they will look like they are dead, but they just go dormant. You can keep them like this for upto about ten days.
The day before you want to use them open the bag and put them in a bait box with some fresh maize flour and any flavouring or spices if you use them and give them a good shake. Put the box back in the fridge with the lid off (if you can get away with it - well ventilated lid if not). They will look all stretched and limp, but will recover overnight and be just as wriggly as normal by the time you come to use them.
I heard this method from Jan Porter, and despite being scheptical I have found it to work blliantly.