Read the whole recipe first before you start. This may sound obvious but I’ve been guilty of it many, many (many) times. You start a recipe, mix half the ingredients and get to, say, step 4 and ingredient 6 and think “I’m sure I had more chocolate chips” or whatever.
Sometimes it’s not practical to put a recipe to the side as you leave to go buy chips or eggs or baking soda or whatever. So before you start, just do a super quick once-over to make sure you have all the ingredients you need. Believe me – I’ve skipped this step before and it’s an enormous pain in the ass.