I know what you're talking about with changes in style and all that. From my experience worrying about consistency is only worth it if you are at a point where you can say 'This is the final design'. (I am aware of the artists habit to withdraw the word 'final' some time later..)
The first step towards consistency is the overall shape of the character or the relative shape and size of specific body parts. The head for example always has the same relative size to the body. Try memorizing the relative sizes of every body part while sketching the character out or create a reference drawing you can peek at (like the ref for one of my characters below for example). Apply it to every body part: Arms and legs have a relative thickness yada yada.
This seems pretty basic, but details like the face are actually built exactly the same way: The size of eyes, nose, mouth and ears is relative to the head, finger thickness = relative to arm and so on. Anything further like eye/nose/chin shape and hair are just a matter of practice. Memorize what the eyes/nose/hair looks like from different angles (once again, a reference drawing helps) and practice until you can produce it without thinking too much.
It all boils down to:
Body parts = be aware of the relative sizes/shapes
Details = practice and memorization
These ramblings encompass what I taught myself at least, so there may be better ways out there. One thing I can say for sure is that reference drawings are a big help even though it is desirable to be able to draw characters consistently from memory. You shouldn't look at references every two seconds, but you should use them to refresh your memory whenever necessary.