Why have intercessions, why not just pray directly to God at all times? First of all, understand that intercessions are optional, they are a tool offered by God to make prayer more comfortable and a little easier or think of it like strength in numbers, several people praying for the same thing works great here on earth and it works just as well if some of the people praying are in heaven. How does one know who to intercede to? To be honest, it really doesn't matter, intercede to whomever you feel most comfortable. The Virgin Mary is a popular person to intercede to because she is the mother of Jesus and therefore the mother of God and all Christians. There are also many patron saints, these are saints whom the church has named "experts" of certain areas because they had a powerful experience in that area during their life time. For example, Saint Anthony is the patron saint of lost articles because of an incident he experienced in which a novice carried off a valuable psalter he was using. St. Anthony prayed very hard that the psalter would be found. After seeing an alarming apparition of him, the novice returned the psalter. As for interceding to our friends and family members in heaven, these are people one turns to during their life on Earth, therefore it makes sense that one should be able to turn to them in heaven as well.
Protestants often accuse Catholics of idolatry because of our frequent use of statues in prayer. Again, this is simply not the case. The statues are meant to guide us into an attitude of prayer. We do not attribute any divine qualities to the statues and we do not ask things of the statues themselves, that would be idolatry.
Matthew Bohrn Senior Web Disciple