Thinning is done for two reasons. First, a certain portion of the fruit is removed so that the remainder will develop adequate size and quality, and, second, the thinning process serves to increase the plant's ability to form flower buds for the next year--provided the thinning is done early enough.
Thin excess fruit when the fruits are the size of the end of your little finger--about 1/2 inch in diameter. If you have a lot of trees to thin, you can start earlier, even when blossoms are still present. Unlike apples and pears, no chemicals are labeled for thinning stone fruits.
Simply start at one end of a branch and systematically remove fruit, leaving one fruit every 6 to 10 inches. Be sure to leave only one fruit at a given site; doubles provide homes for insects and diseases. Keep in mind that only 7 or 8 percent of the tree's flowers are needed to set a full crop of fruit.
Thinning also reduces the total load on the branches and prevents breakage. Thinning is only necessary for apples, peaches, nectarines, pears, and plums; tart cherries should not be thinned.