As far as praying to Mary, a quick explanation is that we ask Mary and the saints to pray for us, just as we ask family and friends on earth to pray for us, because we believe they are our brothers and sisters in Christ, but they're just up in heaven instead of here on earth, so the only way to ask them to pray for us is to "pray" to them, since we can't ask them face to face. That's why the last part of the Haily Mary is "Pray for us sinners, now and at the hour of our death."
DavidMac: What I don't understand is why pray to someone in heaven other than to God directly?
To put it simply, there's no reason we can't pray to God directly, of course we can, and we SHOULD! But the more people we have praying for us, the better, right? Isn't that why we ask our brothers and sisters in Christ to pray for us in difficult times, etc? And also, since the saints in heaven are closer to Christ than we are, and could actually go and talk to Him in person, we believe it's a good thing to ask for their prayers as well.
We believe in what we call the "community of saints," meaning the collection of saints in heaven and on earth who make up the Church, the Kingdom of God. So yes, we are saints now and will be saints in heaven more perfectly, as we believe we will be pure and holy when in heaven.
As for scriptural evidence, in Revelation 5:8 we see him talk about "golden vials full of odours, which are the prayers of saints" while describing a vision of heaven.
Also, historically, this has been a practice since the first centuries of Christianity. Not that this alone proves it's correct, but we often look at the early church fathers to see what they did then since they were much closer to Christ, as far as a timeline goes, than we are, and therefore probably had a clearer picture of what Christ wanted.
More on the subject.
And even more.