You can bake the muffins up to a week ahead of time. Let them cool completely, then wrap in plastic wrap and freeze in resealable plastic bags. Warm thawed muffins, wrapped in foil, in a 300 degree oven, eight to ten minutes. Alternatively, make the streusel and mix the wet ingredients up to two days in advance; refrigerate. The mixed dry ingredients and cinnamon sugar can be stored in airtight containers at room temperature.