Be careful how you motivate. While there are those who point to Matthew 5 as a way of motivating each other to be salt and light in this world, remember: Jesus did not tell us to TRY to be salt and light in this world, He said that we ARE salt and light. In view of the New Covenant, telling people to TRY to be what Jesus thinks they already ARE might well imply that Jesus failed to actually make the change He intended to make, and that people need to become rather than to believe, to change rather than to embrace, to work rather than to rest. That is not Christian.

Through faith in Christ’s cross and resurrection, you have already been made holy (you’re not going to get holier), you have already been made righteous (you’re not going to get righteouser), you have already been made perfect with Him (you’re not going to get perfecter). You’ve been made clean and close, fit with Him forever, just as He intended. Before you go to heaven, there will be no last minute modifications, no bit of re-tooling or re-engineering, and no final scrubbing, because you’re fit for heaven now. You’re heaven ready.

And salt and light? You already preserve and display the handiwork of God, you already add invaluable flavor to a world in need of it, and you already are a beacon of God’s grace in an otherwise dark and frustrated world. He did it. The cross and the resurrection actually worked.
Believing that, and believing it again and again, will provide motivation enough. (For your kids, too.) You ARE salt. You ARE light.

Embrace who you are.