In the same way, the Spirit helps us in our weakness. We do not know what we ought to pray for, but the Spirit himself intercedes for us with groans that words cannot express. —Romans 8:26
Praying in God's will is carried out at one of two levels (or perhaps both): (1) when you know what God's will is and pray accordingly, or (2) when you don't know what God's will is but pray with groans that words cannot express—that is, words you don't understand.
The first level is, in my opinion, quite rare. It has happened, but not often. It is when I know somehow that when I pray for something I also know God heard me and that it will be answered because I prayed in the will of God. I wish I could say that I engage in this kind of praying all the time.
Zechariah and Elizabeth prayed for a son. They did not know that they put this request in God's will and that it was heard. But it was—except that they were not notified for a long time. Zechariah had prayed years before in the will of God, but he was not given the grace of the Spirit to know at the time he was praying in God's will. Knowing at the time is therefore, in my view, rare.
The second level of praying in the Spirit is when you do pray in the will of God, but you do not know what you are praying for. How can this be? Romans 8:26, quoted above, coheres with 1 Corinthians 14:2: "For anyone who speaks in a tongue does not speak to men but to God. Indeed, no one understands him; he utters mysteries with his spirit." When you are praying in tongues you are praying in the will of God, which is why you could pray consciously in the will of God at both levels. This is because when I pray in tongues I abandon my wishes to the Spirit. I do not know what God's will is, true, but I know nonetheless that I am praying in God's will because I am praying in tongues.
Excerpted from Pure Joy (Charisma House, 2006).