1. Before making a move, always ask yourself, "What is the opponent's best response?" If you don't like the results of that response, don't hope that the opponent won't make it, instead look for a better move on your part.
2. Before making a move, always ask yourself, "What does this move accomplish? How does it improve my position or worsen my opponent's position?" If you can't come up with a coherent response, or if the benefits of this move are too slim, then disregard it and look for a better move.
3. Before making a move, always ask yourself, "Are there better moves than this?" Even if you like this move a lot, always look for better ones.
And the main advice, topping these 3: be honest with yourself. Never try to justify a bad move, or a move you made without any particular reason. If you messed up, admit it and try to figure out why you messed up. Only a thorough work on your mistakes can get you far in chess.
Of course, you can only fully exercise this advice in longer games - which is what a beginner should focus on anyway. 
1. Read good chess books;
2. Read some more good chess books;
3. Don't stop to read good chess books until your last days.
(I am not kidding...)