Place the berries in a large stainless steel or enamel saucepan. Bring to a full boil over high heat, mashing the berries with a potato masher as they cook. Add the lemon juice and boil hard for 1 minute, stirring and mashing constantly.
Add the sugar and return the mixture to a boil. Cook, stirring constantly, until it begins to look syrupy and thickens slightly, about 5 minutes. Remove from the heat and test for doneness: a candy thermometer should register 220 degrees F and the mixture should coat the back of a spoon. Run a finger--or a clean object about the width of a finger--through the coat of jam. If the jam does not run and fill the gap, it is done. Otherwise, return the pan to the heat for another 1 to 2 minutes and test again. Be careful not to let the mixture get too thick--it will thicken as it cools.
When the jam is done, transfer it to a heatproof jar and cool to room temperature, about 2 hours. Cover with a tight-fitting lid and refrigerate. The jam will keep for up to 2 weeks.
The key here is not to overcook the jam. It cooks very quickly and chances are that if you think it is thick enough, it probably is. Always remove it from the heat to test for doneness, since the time you take to test it could cause overcooking. Better to err on the side of undercooking, as once it is overcooked--dark, thick, and smelling like caramelized sugar--it cannot be saved.