Here’s a little bit of help from my friends at San Juan Catholic Seminars, on this very subject. Their handbooks are great!
"Why do you Catholics pray to Mary and the Saints when the Bible says Jesus is the “one mediator between God and man” (1 Tim 2:5)?
Q: Why do you Catholics call your priests "father"
when the Bible clearly says “call no man father” (Mt 23:9)?
A: 1 Tim 2:5 must be understood in the light of I Peter 2:5: “let yourselves be built into a spiritual house, to be a holy priesthood, to offer spiritual sacrifices acceptable to God through Jesus Christ.” St. Peter says that Christians share in the one, eternal priesthood of Jesus Christ. Jesus is mediator between God and man because of His priesthood. Therefore, to share in Christ’s priesthood means to share in His mediatorship, both in heaven and on earth.
1 Tim 2:5 confirms that we share in Christ's mediation, when we read it in context. In verses 1-7, St. Paul asks Christians to participate in Christ's unique mediation by offering prayers and intercessions for all men: "this is good and pleasing to God." We are called to unite ourselves to the one mediator Christ, "who gave himself as a ransom for all," by praying for all men, through Christ.
Because Christians share in the priesthood of Christ, we share in a lesser and dependent way in His unique mediation, interceding for all men.
Fellow Christians on earth intercede for each other in prayer without contradicting the unique mediation of Jesus Christ. Likewise, there is no contradiction of 1 Tim 2:5 if the saints in heaven intercede for us with their prayers. All prayer, whether in heaven or on earth, is in Christ and through Christ, our one mediator and high priest.
The principle is this: although God alone possesses all perfections, we can participate in God's perfections by sharing in His divine life. For example, the Bible says only God is good (Mk 10:18). Yet we can share in that absolute Goodness: "Well done, my good and faithful servant" (Mt 25:23).
Jesus shares many of His unique roles with Christians in lesser ways. Jesus is the Creator of all things (Jn 1:3; Col 1:16-17), and yet He shares this role with men and women in procreation. Jesus is the only Shepherd (Jn 10:11-16), yet He delegates this role to St. Peter (Jn 21:15-16) and later to others (Eph 4:11). Jesus is the eternal High Priest, mediating His once-for-all sacrifice for our redemption (Heb 3:1, 7:24, 9:12, 10:12), and yet Christians are also called to join in Christ's priesthood, as we have seen (1 Pet 2:5; Rev 1:6, 5:10).
Obviously, Christ is the unique and primary Creator, Shepherd, and Priest, but each Christian participates in these roles in subordinate ways. By sharing Christ's divine life, Christians also share in Christ's role as the only mediator.
That’s from their site.