I have a lot of 2dcg 'rules of thumb'. Many of them are much less useful outside of context, but you might glean something from them. But remember they're rules *of thumb*. Thumbs are opposable.

Proportions

Barring stylized proportion, hands should be big enough to facepalm.

	Hair is obscuring more than you think.
	The body folds down like an accordion, in 3 parts (torso-thighs-shins), roughly.
	Facial features occupy a fairly diminutive triangle on the front of the face.
	Barring perspective or style, heads are not as big as torsos.

	Fat bulges. Tendons pinch. Bones chisel. Knuckles are very chiseled and pinched.
	The mouth is round. The general eye socket area is round. Both are flatter than expected.
	Faces aren't perfectly symmetrical, and there's no impetus to make them that way.
	Features never touch the silhouette of the face unless they have a very good reason to, because it is cold and scary out there.

	Pick a camera angle first.
	Technically, the field of vision stretches out into a sphere. Pick an area and stick with it.
	Key the perspective with important scene elements. Work outward.
	Don't fucking draw boxes around shit.
	If it's overlapping, it probably isn't overlapping enough.

	When in doubt, pick one: Dodge/burn/levels/saturation.Try to avoid this situation altogether by palettework.
	Photoshop user: It's probably too desaturated, whatever it is.
	Photoshop user: Avoid mixing one color straight into another because PS's blending algorithm is evil and untrustworthy. Put another in the middle of the gradient (pref. a more saturated one, barring very opaque materials)
	Mind the edges. Accidental color banding is as much of an issue outside of pixel art as it is in.
	It's probably mixed enough. Don't kill edges.

	What are you trying to communicate?
	Lean.

Silhouette first. Clothes are too complex to draw in positive space; use negative space instead.

	You know what you're doing better than anyone else does, probably.
	If it looks weird, give it a bit to see if it pops into place with the rest of the render. If not, fix it.
	Take a break.
	Take a nap.
	Eat a substantial amount of food.
	If in physical pain or panic, Stop.
	Don't compare wholes; dissect.