This number one rule of spells works with the 'like attracts like' principal. So that when you spell cast - you must believe, know, feel, see that what you want to have happen will.
This is where is can be problematic for people. As we are so full of the emotion - it might be worry over money, or heart-ache over lost love. And that feeling is incredibly prevalent, and it requires a huge commitment, conscious decision and hard mental and emotional discipline to choose to focus on what it is you want as if you have it now.
So whilst you cast the spell - and a spell in, of, itself is indeed powerful - you need to be focused on what you DO want. And then even after casting that spell, to support it happening - it requires you to move into the emotional state of certainty that what you want to have happen will. Many would call this faith - and faith is incredibly powerful!
"Reality is wrong. Dreams are for the real."