Websites such as Maximum Generosity by Brian Kluth, What Christians Want To Know and Lords-prayer-words.com provide sample prayers to be used for offerings. Many prayers focus on acknowledging God for faithfulness and asking God to help everyone give cheerfully and purposefully.
A sample prayer at Lords-prayer-words.com acknowledges the Lord's faithfulness, praises God that he is trustworthy and merciful, and acknowledges that giving the offering is a form of worship to be used towards God's kingdom.
Sample prayers from What Christians Want To Know have themes such as asking that all give generously and that God bless the offerings, praying for individual blessings when one is obedient and praying that God be honored by man giving back to him.
Some additional prayers that Brian Kluth suggests include praying to grow both spiritually and in generosity, to give thanksgiving in what one has and praying that God can turn little into much.