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human
I'm no expert on the topic, nor very experienced but what helped me to find that conviction was looking back and finding moments in time where my 'prayers' where answered before I knew of the Law of Attraction and before reading Goddard's lectures.

Of course, you might not have any memory of something like this happening or maybe you cannot tie the ends together. 
In that case try to make small steps, desire something that you feel comfortable desiring, something that isn't too outrageous. Look again at the ladder experiment, try to create something similar, something that isn't too hard to desire.
When you sincerely try to prove (or disprove) the law, you will find that it actually works.

Hope that helps a little bit, I know that finding true conviction is the hardest part of it.
gpt
Oh yeah, I can definetely think of examples where I thought of something and had it happen to me later