Been there done that several times. Lot to keep in mind while doing, don't pick up or disturb items, call over the evidence collector to photo and collect evidence. Don't leave or wander from the pointed out area. Don't make a game of it, the police will be there watching you. Don't add to the scene (litter). After you are finished and think you are completely done, BE PREPARED TO GO TO COURT to testify. Good jobs result in plenty of call backs. You will most likely only be dealing with 1 or 2 detectives and possibly 1 or 2 police officers, on large departments the patrol officers will not know you even helped in the evidence retrieval so don't expect favors. Be prepared and conduct an organized gridded search pattern with your helpers, police usually expect several persons that are very expert with the detector, and to act professional. If you aren't prepared for the above then don't even offer. That's my best advise.