When getting ready to cook or bake, I get every ingredient that I will need out of the cupboard. I set them all out on one side of the bowl where I will be working. Then as I use each ingredient, I either move them on the other side or put them right back in the cupboard. I get appropriate measuring cups and spoons out right at first too, so I have everything that I need right at my fingertips.
This tip accomplishes two things: I always know what ingredients I already put in the recipe(sometimes I just forget!) and before starting to cook, I can take stock of my ingredients and make sure I have everything before I am in the middle of mixing and realize that I don't have enough of something! This really reduces quick trips to the store.:-)