always look at/double check the different two-dimensional relationships. depending on what you're drawing, and even the particular instance (say, of the head), these can differ, but for instance the vertical value of the side of the nose relative to the bottom of the ear. if you don't know how to make comparative measurements, learn. Keys to Drawing has a good guide, but there are others; just watching demos of trad artists can be useful in that regard too (for me, Stephen Bauman).