Although metal detecting may be fun and all.. I have a simple suggestion here. When it's raining, or snowing, people drop plenty of coins and paper money in parking lots reaching for their car keys. No body likes crawling under a car when its' wet to grab a dropped bill, nor have I ever seen anyone dig in the snow for dropped coins. Instead of metal detecting and digging in dirt.. just wait for it to stop raining, or snowing and go out and pick up the dropped money.
I used to sweep store parking lots at night with a truck, and I always made sure after a hard rain, or snow storm to check the curb and grass line on the downwind side of the parking lot. I only did five stores a night, and my best night during Christmas week was around eighty bucks FREE money.
The storm drain grates in front of bar rooms are another great place to look for LOST money ! If you are worried about being harassed by the cops, or business owners, just put on a yellow safety vest and a white construction helmet (which you'll find pretty inexpensive in a second hand store).. It's a lot cheaper than a metal detector anyway.
Hope these tips will increase the profitability of your detecting. Good Luck.