Preheat the oven to 375F.
In a large bowl, cream the butter, sugar and lemon zest together with a spatula. (If you beat the mixture for awhile with an electric mixer, the muffins will not rise - stick to doing it by hand.) Add the eggs one at a time, mixing after each.
Add a cup of the flour along with the baking powder, soda and salt, and mix well. Add half the yogurt and stir until combined, then another third of the flour, the rest of the yogurt and the rest of the flour. Stir in the berries, or whatever fruit you want to use.
Divide the batter among muffin tins that have been lined with paper liners or sprayed with nonstick spray, filling them. (You'll get about 18-20 muffins.) Sprinkle the tops with sugar and bake for 25-30 minutes, until golden and springy to the touch.