Interestingly enough, Jesus answered this very question. In Matthew, as he began teaching, he outlined a way of praying that we call "the Lord's Prayer." In opposition to the wordy, lengthy and complex prayers done by religious leaders to impress others, Jesus taught a simple way of praying. He said, "Pray then like this" (Matthew 6:9-13).
Jesus gives the same teaching another time, as recorded by Luke, when his disciples specifically asked him to teach them to pray. In Luke 11, Jesus teaches "the Lord's Prayer" and gives it to his disciples as a legitimate prayer when he says, "When you pray, say" (Luke 11:1-4). If we pay attention to Jesus, he teaches us how to pray. Luke helpfully recorded four teachings of Jesus about prayer. He combined them into two sections in the Gospel of Luke: 11:1-13 and 18:1-14.
In Luke 11, Jesus teaches a model for prayer that can be used and then teaches about persistence in prayer by encouraging disciples to ask, seek, and knock for whatever they need. Jesus encouraged his disciples in prayer by reminding them of the presence of the Holy Spirit.
In Luke 18, Jesus tells two parables about prayer to remind us once again to be persistent, that is in God's character to provide justice to his people, and to be humble in prayer as we go before God.
Learn from these lessons. God desires communication with him to be simple. We make it complex when we think more about how our prayers are received by others, or when we ask whether they are long enough, whether we used the right words, or if we have asked too often for the same thing.
Jesus gave us a model of prayer to use. We should use it! We should remember the power and presence of the Holy Spirit in prayer. We should be reminded of God's desire to bring about justice for his people. And we should always cultivate the inner attitude of humility that God calls us to have.