"And all things you ask in prayer, believing, you will receive"
This is an amazing verse which echoes what Jesus has said at other times. The ability to activate prayer is belief. Your mind must be able to embrace the truth of what you are asking for before God will answer the prayer.
Jesus says that if you cannot conceive of what you are asking for in prayer actually coming true, then it will not come true. The activating mechanism for prayer is your mind's ability to construct and hold the truth in the future of what you are asking for.
Believe: this is the Greek word pisteuo, which means to put one’s trust in, to think to be true, to be persuaded that.
The question arises: how does one come to the place of belief?
One can be persuaded by the evidence or facts of the case which is this culture's preferred way of coming to belief regarding something.
One can be persuaded by an authority or trusted source allowing the mind to conceive of the thing as completely true and trustworthy.
Belief can also be a function of the mind completely. Our minds can concoct and hold an idea that is not real yet but may become true in the future. This is how all new inventions and ideas are developed. A person conceives of what has not been conceived of before and then conceptualizes it in such a detailed way that they are able to then build it or develop it. It was their belief in the concept in their head that allowed it eventually to become a reality.
Some have suggested that you have to go into the future in your mind and see the future with what you want having already taken place. Embrace the scene, feel the emotions, absorb the situation and the facts, smell the smells, and see the sights that will come about when this thing is true. It is holding that image and that future that is the essence of belief. If you cannot hold that scene in your mind as you are praying, then it will not be received; not until you already have embraced it as true when you ask for it that you will receive it.
There are other limitations mentioned in other Scriptures such as is it selfish, is it in God's will, etc. But Jesus here is sharing a secret of answered prayer -- you must go into the future and conceive that thing you want to pray for as already true.