One other thing, never brag about what you have found in the past. You are more likely to receive permission if the property owner perceives you as a nutty coin collector searching for coins and tokens of little monetary value than if he views you as a wildly successful treasure hunter about to make another fortune off of his yard. You can reinforce the "poor coin-collector" image by carrying a handful of really ugly looking coins that you have found in the past. I find Liberty nickels to be good show pieces. Unlikely that the property owner has seen one before, so they are interesting but ugly coins with little or no apparent monetary value.

Eventually you will graduate from vacant lots to domestic yards. I have a bit of luck in stopping to ask when I see an older lawn being dug up in preparation for reseeding. Rototillers can be a coin shooters best friend (and worst nightmare, there are some unfortunate scratches on the backside of this 1896-s Barber quarter, sob, sob!)