Don’t pray to dead humans!
Well, good advice there.
But the Saints are alive in Christ: far more alive than we can currently understand.
Remember that to pray is simply - to ask. "I pray thee" means the same as "Please".
If I can ask my friend Gary for help with my car, why can't I ask Mary to pray for me and my family?
Lastly: the prayers of the Saints are so important that they get a specific mention in the Apocalypse.