Hey man, you should try establishing the general proportions before adding value. Just do a very simple block-in of your subject before committing to adding in the value and make sure the proportions are spot on before you start on anything else. You want to fix those big mistakes early on, then you can start blocking in the value (zooming out and flipping the canvas can help figuring out where the mistakes are) and make some final adjustments to the proportions if necessary.
Make sure you picked the right values too (I suggest using as few as possible and keep them on one layer to sample from, something like 3-5 with values ranging between the darkest dark and lightest light you intend on using). Then you can start finishing things up, try to nail the transitions and make everything look good.