Quite popular: 'Faith is the substance of things hoped for, the evidence of things not seen', and I really do hope it's not just a chant, but something we practice. Naturally, it's quite difficult believing something that is obviously (physically) not existing, and the devil does not make the process of faith any easy, when he keeps on helping you to see and reason the physical obvious.
So how do we improve our capacity to trust God?
The first and simplest method is by filling yourself with the Word of God (Rom 10:8) and thinking it. Here, the saying that knowledge is power definitely applies. This is simply because what you do not know you have, or what you are not sure you have is difficult to claim confidently. This would help your confidence and would help you to counter any form of doubt that the devil might bring your way (Matt 21:21).
You should also speak that which you believe (2Cor 4:13). "You need to stop describing your present situation and start declaring your future expectations", Bishop David Abioye. There's just something about what you speak, it pulls unknown forces together to bring it to pass, and that it why it is said, 'say not before an angel that it was an error. Therefore, make positive confessions, speak the Word.
Also, it is definitely important to walk in love (1Cor 13:13). This is because anything that has to do with God has to do with love, for God is Love (1Jn 4:16). In this, it is love for God and love for those around you. With respect to God, it implies obeying Him and keeping His laws, for love is the fulfillment of the law (1Jn 5:3, Jn 14:15, 1Jn5:2). This way, the devil has nothing to hold against you before God and besides that, sin does have a way of reducing your confidence. With respect to those around you, it implies forgiveness (Ps 66:18, Matt 18:21&22) and generally walking in love.
Sometimes, we do need the extra help of the HolySpirit, this is what is referred to as the gift of faith. This is when you do ask the HolySpirit to help strengthen your faith like the man here: Mark 9:24, or sometimes, He just does it to perform a miracle. For example, passing an exam you did not sit for due to an illness.
It takes faith to get it all. God bless you.