Homemade buttermilk: put 2 tablespoons of white vinegar or lemon juice in a measuring cup and add milk until you reach the 2-cup mark. Let it stand a minute or two until it curdles and thickens, then use it in the recipe.

Baking time: remember that all ovens and pans are different, even if they look similar. The baking time in my recipes is as accurate as it can be, but it might take you more or less time. You can use a thermometer(like the OXO oven thermometer) to check that your oven is at the right temperature. I recommend you keep track of how your oven works and what tiny details you might need to adjust.  

Ingredients: don't substitute them; they are all necessary for the best results.

Sift the baking soda: it tends to harden when stored, and you will probably find clumps when you measure it. Sifting before adding it to the flour will ensure it's well integrated. This is very important, so don't skip it.

Overmixing the dough: Brown soda bread dough should only be mixed until the ingredients are just combined. Overmixing can lead to a tough, dense bread.

Dough too dry or too wet: the texture of the dough is important for the success of your bread. If the dough is too dry, the bread may turn out crumbly and dense. If the dough is too wet, the bread may not rise properly and may have a gummy texture. You should be able to form a round loaf and make the top cut. If in doubt, always err on the side of a wetter than drier.

Oven temperature: this bread needs to be baked at a high temperature (around 200-220°C) in the beginning to create a crusty exterior. If the oven temperature is too low, the bread may not rise properly and may be undercooked.

Eat it the day it's baked, preferably within the first hours. I strongly recommend this. This no-yeast bread hardens quickly. If you want to freeze it, do so while still barely warm and wrap it well in plastic and then foil. I also recommend slicing it first to have individual toast whenever you want.