Personally I prefer reds in the summer and bronze in the winter. A good indication of freshness is the size of the 'feed spot', this is a black 'blob' of the maggots last feed which can easily be seen within the body of each maggot. Look for large soft maggots with a large feed spot to guarantee your bait is fresh. As for preparation, once you get your maggots home, riddle them off using a maggot riddle and remove all the medium that the maggots were stored in along with any 'dead' skins. Once riddled, place them in a clean container with a handful of either fresh maize meal, sawdust or bran.
Custard powder is a secret kept in many match anglers cupboards.
These are mostly obtained by 'home breeders' but their treatment is much the same as for normal maggots. These are a slightly smaller maggot and are the larvae of the common greenbottle, so called because in their uncoloured state they have a 'pinkish' tinge. These can be used as hookbait using a smaller hook when the fish are proving hard to catch, or, as a loosefeed when using normal maggots as hookbait. Again the same rules apply re freshness and preparation as to the larger maggots. The name is probably due to the fact that these are the smallest of the maggots used in fishing, they are the larvae of the common housefly, sometimes also referred to as 'feeders'.