How do I know what God wants me to do? Must I have a "mountaintop experience" for every instance where I need the leading of God's specific will?
The point of mountaintop epiphanies - the Transfiguration itself as well as the experiences we as Christians occasionally have with the palpable nearness of God's presence - is not to stay always in their blazing glory. Instead, as the text shows, the point is Jesus' death and resurrection (a greater glory) that assures of freedom in God's personal presence always. This freedom is found in the Lamb of God, Who takes away the world's sin.
We are God's baptized children by faith. As children, our duty is to "listen to Him!" How do we do that? Meditate on the Bible - and live it out. It is both the story of God's love for us and instructions on how to live. The Spirit will give understanding as we read the whole Bible, time and again.
The Holy Spirit always points us to Jesus, through the Word, relationships, and our hearts. Regarding relationships, we serve our neighbor in the unified Body of Christ, whatever his or her need. We serve our enemies, too, risking their rejection but not fearing it whatsoever. All things weigh against Jesus. Regarding our own hearts, consider Psalm 37:4. He gives us good desires to displace our evil ones.
Lastly, consider your gifts. If they're God-pleasing desires, then pursue them and watch what God will do with them! "Without Me you can do nothing" (see also Psalm 127).Soli Deo gloria.