“If you abide in Me, and My words abide in you, you will [a] ask what you desire, and it shall be done for you.” John 15:7

This is Jesus saying to you, that if you consistently do two things, He will give you all that you ask of Him in prayers: These two things are, “Abide in Me”, and “Let My words abide in you”.
       This promise is assuredly true because it is impossible for God to lie (Heb. 6:18). So we can rely on it when we pray. All that we have to do is meet those two conditions:
   
1. Abide In Me: What does this mean? To abide in Jesus means to live in Him, to stay in Him, to remain in Him or to continue in Him. In other words, it means to live your life totally dependent on Jesus; allowing Him to guide you through the scriptures and through Him revealing things to you. You in turn obeying every word He leads you to do. As a Christian, you will be very successful only by abiding in Jesus. Of course, be aware that He will more often lead you through His Holy Spirit who dwells in you, and so the process of abiding in Jesus, implies being led by the Holy Spirit, and you yielding to and obeying the Holy Spirit. 
 
2. Let My Words Abide In You: What does this mean? To let the words of Jesus abide in you means to retain much of His words in your heart; letting the words dwell richly in you. And how do you do that? You can do it in three ways. (i) Memorize the scriptures, (ii). Meditate on them till they stick to your mind, and (iii). Recite them many times until they stick to your mind. There are several very beneficial reasons to retain the words of God in our hearts, but in respect to answered prayers, we note that the Holy Spirit requires the words as His sword or power weapon to pray and fight for us so God can answer and grant our requests. Furthermore, He prefers to use the words retained in our hearts to do so.
       So, if we so live our lives totally dependent on Him, and fill our hearts with His words, then we should expect Him to step in and grant all our requests by promise. Amen!