When a Catholic says he is "praying" to an angel or saint he is using the term prayer in the sense "to ask." It means to petition someone for something. The Catechism of the Catholic Church (CCC) words it simply to "ask them to intercede."
CCC#2683 When they entered into the joy of their Master, they were "put in charge of many things." (cf. Mt 25:21) Their intercession is their most exalted service to God's plan. We can and should ask them to intercede for us and for the whole world.This example is seen in Scripture many times when someone asks an angel or another member of the Body of Christ for intercession.
- The Psalmist asks the angels to join him in prayer (Ps 103:20-21, Ps 148:1-2).
- The Israelites consult the angel who intercedes to God for them, and God responds to the angel's intercession (Zec 1:11-16).
- Paul asks other members of the Body of Christ to intercede in prayer for him (Rm 15:30, Col 4:3, 1 Thes 5:25, 2 Thes 3:1).
- The elders and angels in heaven are seen passing on the "prayers of the saints" (Rv 5:8, Rv 8:4).
This interaction between members of the Body of Christ is a testament to the unity of the Body of Christ. Both the members in heaven and earth are a part of the same family in Christ (Eph 3:14-15). Whether it's an angel in heaven or our neighbor next door, we can ask them for intercessory prayer.
So the principle is set forth in Scripture: We can ask for the intercession of another member of the Body of Christ without violating the singular mediation of Jesus Christ, nor granting the saint the worship due to God alone. The principle finds no qualification in Scripture that asking members of the Body of Christ in heaven is off-limits. In fact, Scripture tells us the prayer of a righteous person is most powerful (Jm 5:16). All the more should we then want the intercession of those fully united with Christ!
Graphically, praying to saints (or angels) looks like this:
Notice how the ends of intercessory prayer is always "God." When we pray (i.e. ask) a heavenly member of the Body of Christ for intercession, we are not going to them "instead of" God. Certainly we would not have considered Paul to have gone to the Romans "instead of" God when he asked them to pray for him, no? The same holds true for asking a heavenly member of the Body of Christ to pray for us.
Therefore, intercessory prayer neither violates the unique salvific mediation of Jesus Christ, nor applies worship to a creature.
Another concern that may come to mind is "How can the dead hear us?" Doesn't Scripture say the dead "know nothing"? (Ecc 9:5)
First, we see in Scripture a number of examples of the conscious awareness of angels or those who have undergone physical death.
- Tabitha, who was dead, rises at the prompt of Peter telling her to rise (Act 9:36-40).
- Jesus speaks with "dead" OT saints Moses and Elijah (Mt 17:3, Mk 9:4, Lk 9:30) (Granted someone could say Elijah didn't die but was assumed, but going by Scripture the same cannot be said of Moses, cf. Josh 1:1).
- The angels are fully aware of what we say and do, and they even rejoice when a sinner repents (Lk 15:10, 1 Cor 4:9, Ps 91:11).
- I would also reiterate the vision in heaven of angels and elders passing on prayers mentioned above from the book of Revelation. And even in the Old Testament, although done in a sinful way, a witch summons a manifestation of the deceased Samuel, which she could not do if the dead could not respond to her (1 Sam 28:7-14).
So what does the "dead know nothing" mean in Ecclesiastes? The very book can be confusing and one should not jump to form doctrine from contextless verses therein. The book itself opens with most non-doctrinal words: "All is vanity!" Ecclesiastes 9:5, which says the dead know nothing, also says there is no memory of them, which of course could be disproved simply by remembering someone dead. In verse 7, the speaker says "Go, eat your bread with enjoyment, and drink your wine with a merry heart..." This concept is one Paul actually condemns when writing to the Corinthians: "If the dead are not raised, let us eat and drink, for tomorrow we die" (1 Cor 15:12). But of course the dead are raised as Paul argues, which suggests the writer of Ecclesiastes could only be correct (if he was ever correct) if there was no Resurrection.
Furthermore, the speaker in Ecclesiastes goes on to say that these "dead" are going to "Sheol" (Eccl 9:10). This is commonly held to be the "abode of the dead" (Strong's Concordance #H7585 , CCC#633, etc...) where Jesus visited and preached to "the spirits in prison" (1 Pt 3:19). If one wishes to consider the dead in Ecclesiastes to have been in this "abode of the dead" or whether he wishes to consider Sheol by it's only other definition of "hell," neither applies to the heavenly members of the Body of Christ in the New Covenant. Therefore, one cannot use Ecclesiastes to say the heavenly members of the Body of Christ "know nothing."
Finally, let's say one were to jettison all of the above and still insist he is not comfortable asking anything from anyone in heaven except for God. A solution for such a person would be to ask God to pass on his request for intercessory prayer. For example one could say: "Dear God, please ask Elijah to pray for my illness." In doing so, one addresses only God, yet still follows Paul's example of prompting other members of the Body of Christ to pray for him!
EDIT: September 6, 2010
Supporting the idea that Ecclesiastes 9:5 refers to those in Sheol is:
Psalm 6:5 For in death there is no remembrance of thee; in Sheol who can give thee praise.