The advantage of doing the prep work ahead of time (let's remember that "prep" is short for "preparation," not "preparatory," as in "preppy") is that you are far less likely to forget an ingredient in the heat of the moment -- and who among us has not, at one time or another, forgotten to add some small amount of an essential ingredient? Salt, herbs, lemon zest -- or, worse, baking soda or powder -- if they are sitting there in a row with the other ingredients, you will remember them. It is also much faster, in the final analysis, to do all the measuring at once, and then be able to add things in quickly as you mix. If you are working with a hot mixture on the stove, and time is of the essence, mise en place for at least a few ingredients might be required.
Small custard cups (like the ubiquitous Pyrex ones) are good for mise en place. I recently bought small multi-colored silicone cups that will be useful, as well as a set of stainless-steel Indian cups specifically designed for Indian cooking, which uses many different spices, often added at different times. You can use cupcake papers or silicone cupcake cups, which also come in many different colors (useful if you have ingredients, like baking powder and baking soda, that look identical but get added at different points). Of course, for larger amounts, use larger bowls or measuring cups. Don't use a cupcake pan; how will you get one ingredient out while leaving the others in their respective cups?
I've been resistant, myself, to mise en place, but Betty has shown me the True Way, and I've come around to it. You may end up with a few extra little bowls to wash -- but that's better than ending up with a ruined cake, or racing around the kitchen looking for the vanilla while your cake over-mixes. Be Prepared!
* pronounced "MEESE ehn plahs."