The people who hear God's voice best are the ones who know his word most.
Jesus said to his followers, "If you hold to my teaching, you are really my disciples. Then you will know the truth, and the truth will set you free" (John 8:31-32). If we hold to his teaching, meaning live in it, abide in it, and follow it, then we will know the truth. That's exactly the way hearing from God works. When we abide in his word, we will know his voice, which is truth.
All of scripture is God talking to his people. So, in order to hear from God, you must know scripture.
God has given us scripture, and he calls it his word. It is the word of God. And we refer to it as God's word. So, how do you know when God's talking to you? Because he talks through his word.
In order to hear God talking to you, you must know his word. The more you know his word, the better you'll hear his voice.
Giving is the only antidote to greed.
Jesus says, in Matthew 6:24, No one can serve two masters. Either he will hate the one and love the other, or he will be devoted to the one and despise the other. You cannot serve both God and Money. Jesus is making the point that our heart's highest loyalty will be for only one thing. We can't be equally devoted to two things; ultimately one will win out over the other.
If money is the most important thing to you (i.e., if you're greedy), then you will serve your money more than you serve God. And you will find it very difficult to follow God's instructions to give to others: If there is a poor man among your brothers ... do not be hardhearted or tightfisted toward your poor brother. Rather be openhanded and freely lend him whatever he needs (Deuteronomy 15:7-8).
If it's true that no one can serve two masters, then the best way to fight against greed is to make sure that your master (your highest loyalty) is God—not money. Follow God above all else; that way your money will follow God, too.
John writes, If anyone has material possessions and sees his brother in need but has no pity on him, how can the love of God be in him? (1 John 3:17). That sounds rather harsh at first, but it's really no different than what Jesus said in Matthew 6:24. If you love your money more than God, then you'll keep your money for yourself rather than giving it away to help others. The only way you'll give generously to others is if you love God more than you love your money. So, how do you know if you love God more than money? By examining what you do with your money: Do you keep most of it, or give most of it?
If you want to fight against greed (that is, if you want to love God more than your money), then you'll follow God's instructions to give—give to the poor, give generously, give joyfully, give liberally, and give regularly. Giving is the best weapon against greed.