What i've always found interesting when 'coaching' younger or weaker players is the old maxim; If you want to really understand what it is that YOU know about something, try teaching it to someone else. I've gotten much stronger by giving advice at my club, as after a while they start coming right up to your level and you have to find new ways to teach them more. meaning that i am forced to learn in order to instruct.
Critical moments help please
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When you develop your pieces, if you put one on the wrong square, usually you can fix it later, albeit at a loss of tempo. When you're in a critical moment, you should have the feeling of "Okay, I -have- to get this move right"...they're critical moves that often determine the outcome of the game. Often when the center opens up, that's one critical moment. When playing a Ruy Lopez, the entire game is often based around the central tension and which side can release that tension to their own advantage.
When you make a move that commits you to a long term plan at the expense of another, that's critical, because you can't go back and fix it if you chose wrong. You're already committed.
Remember that all moves are critical. Some are just more critical than others.
As for where to develop your pieces when your opponent leaves theory, I'll give you two pieces of advice, one easier to implement than the other. The easy one is to maximize the number of squares your pieces can control/attack, especially central squares.
The harder piece of advice is to have a plan based on the structure that you see forming, and understand what your plan needs to be, and what your opponent's plan should be, and place your pieces to advance your plan while hindering your opponent's.
I'll throw you an unrated challenge if you want some practice and more discussion.