We invoke saints to pray for our needs the same way we ask our friends and families to pray for us. In James 5:16, St James says that the prayer of righteous people are powerful in their effect. Those in heaven are the most righteous and their prayers are more fruitful. St Therese of Lisieux, also known as St Therese of the little flower, said before she died that if she went to heaven the fruits of her intercessions for us would be like a shower of roses coming down from heaven. Many people have benefited from her efficacious intercession, including my father as I described here.

Here is a good prayer to invoke St Therese to pray for our needs.

Miraculous Invocation to Saint Therese of Lisieux

(Recite daily in sickness and trials, especially when all help seems lacking)

Little Flower of Jesus, ever consoling troubled souls with Heavenly Graces, in your unfailing intercession, I place my confident trust. From the Heart of Our Blessed Savior, petition these Blessings of which I stand in greatest need (mention here). Shower upon me your promised Roses of Virtue and Grace, dear Saint Therese, so that swiftly advancing in sanctity and to perfect love of neighbor, I may someday receive the Crown of Life Eternal. Amen