To thicken a brown sugar glaze, you can simmer it over low heat to reduce the liquid content and naturally thicken the mixture. Stirring occasionally helps prevent sticking or burning. Alternatively, you can create a slurry by mixing 1-2 tablespoons of cornstarch or flour with a small amount of cold water. Add the slurry to the glaze while stirring continuously, and heat the mixture over medium heat until it reaches the desired thickness. Another option is to use arrowroot powder in a similar manner as the cornstarch or flour. Gradually add the thickening agent and adjust the amount as needed to achieve the desired consistency.