The doctrine of God's omniscience touches so many practical aspects of our Christian lives. I'm not sure how many of these I will write on, but I really wanted to say something about God's omniscience and our prayer lives...
Some might say that if God already knows what will happen, we don't need to bother praying. Well, that would certainly fly in the face of God's clear commands to pray - ie. "pray without ceasing".
But, understanding the omniscience of God should effect how we pray. Jesus taught that "When you pray do not heap up empty phrases as the Gentiles do, for they think they will be heard for their many words. Do not be like them, for your Father knows what you need before you ask him" (Matthew 6:7-8). Moreover, I think we can rest confidently that God knows better than we do what we need. After all, he made us and knows the plans he has for us.
So our attitude in prayer isn't coming to tell God what we need - he already knows. Certainly we are to bring our requests and need to God in prayer, but not to inform him of something he doesnt already know. We bring our needs to him as an expression of trust and dependence upon him to meet these needs.
I find that reassuring, because there are so many times when I frankly don't know how to pray. I don't know what I need, what the situation calls for, what would be best. But God knows before I ever utter a word.