Here's a few bits of wisdom I learned from the elderly gentleman who sold me my first detector 40 years ago. They go in order and each one increases the probability of finding good targets over the last one.
1. Anywhere people have been, they have lost things.
2. The more people who have been at a given location, the more items that have been lost.
3. When people are distracted they lose more things.
By following that logic you have a good start on being successful. The best sites are those where a lot of people had their minds on something else. You might find something in the middle of an open pasture but the finds will probably be few and far between. But an old pasture where they used to hold carnivals is a different story. Some of my favorite sites to hunt are churches, the older the better. I approach the minister and explain what I want to do. I promise to not damage the property in any way. Then I tell him that I will notify him of any jewelry found so that he can tell the congregation. If a member calls me and describes the item I found I return it to them. I recently found a high school class ring that had been lost in 1980. I tracked down the owner and returned it to him. The other day I found a child's ring and a sterling silver men's wedding band within a few feet of each other along the walkway between the parking lot and the church. I'm waiting to see if anyone claims either of them. Doing this not only opens churches for me to hunt, it is great PR for metal detecting in general.
HH,
Storm