If your job will take more than one gallon--and most will, it seems--when you have used half of the first gallon you bought, refill the first can by about a quarter from the second can. Keep doing that so you are always pouring into your paint pot and/or roller tray from the first can that has half from its current contents and a quarter from the next can. This will minimize any variations in color from one can to the next. Also, do use a paint pot, a small pail with no lip, that lets you carry around a smaller quantity and you do not have to fight as much to keep paint out of the lip where the can lid fits. If you are using a roller, you will still have to have a brush for the corners.