Sprouts form in lower leaf axils and then continue to develop and mature upward. When sprouts mature, nearby leaves turn yellow. Harvest buds when they are small and tight, about 1 to 1½ inches in diameter. Break or cut off yellow leaves above developing buds as you harvest upwards. Remove leaves just above buds a few days before harvest leaving about 2 inches of leaf stem on the stalk as you remove each leaf. This will give developing buds room to grow round.
For more information on growing Brussel Sprouts, check out this great guide from Harvest To Table: