To remove burned sugar from a saucepan, put just enough water in the bottom of the pan to cover the residual sugar, and then return the pan to the stove and allow the water inside to simmer. The sugar will gradually melt away from the bottom and sides, so there's less mess to clean.
Because the water dilutes the sugar, it is not necessary to finish cleaning the pot right away. As an added bonus, add any spoons or utensils to the water to remove residual sugar from those objects as well. Particularly tough sugar residue can be soaked in hot water in the sink for 30 minutes to expedite the cleaning process.