This is my current favourite blueberry muffin, another low fat recipe using yogurt. #cookingathome 

I kept a couple for 5 days in an airtight container and it was still soft and moist.

I'm not actually on a health kick. The truth is that butter is expensive and yogurt was on sale so I'm totally rocking the yogurt as a fat substitute in baking. I suggest using full fat yogurt unless you're counting calories (I'm using 3 percent because it was available). You won't get the same flavour without butter but these are pretty good.

Also, say what you will about bleached white flour, but I can't be bothered to buy more than one type of flour. The one I buy can be used to make a passable substitute for cake flour AND it works as a decent bread flour.

NOTE 1: I reduced the sugar to 2/3 cup because I prefer less sweet, replaced the butter with 1/4 cup of yogurt and used 1 1/4 cup of frozen blueberries but 1 1/2 cup is probably better. Don't thaw the blueberries, just add them right in.
NOTE 2: I added 1 tsp of cinnamon. I've tried it with and without the cinnamon and I like it better with cinnamon.
NOTE 3: I don't believe in nonfat dairy. The yogurt I used is 3% fat and I think it helps to make up for the missing butter.
NOTE 4: Comments for my favourite low fat banana muffin recipe warn that they stick like crazy to paper liners. I assume the same will happen here so I bake these in silicone muffin cups to ensure clean release.

Recipe is here - http://www.tasteslovely.com/moist-blueberry-muffins/