Disciple: “What must I do to be prosperous?”
God: “Be strong and courageous!”
Disciple: “Strong and courageous for what?”
God: “To be steadfast (not turing to right or the left) in observing all that I have commanded.”

Points to ponder:
Wanna be prosperous? Be steadfast in observing what God has commanded you to do, without weakening under the pressures of a world that is in enmity with God (James 4:4). To obey him is to love him and to love him is to keep (observe) his commandments, for Jesus said if you love me, you will keep my commandments (John 14:15), and in keeping his commandments will all men know that we are his disciples (John 13:35). Would you consider yourself as prosperous? In other words, “Are you being strong and courageous in being steadfast – steadfast in keeping God’s commandments?”

Joshua 1:6-9 (KJV)
Be strong and of a good courage: for unto this people shalt thou divide for an inheritance the land, which I sware unto their fathers to give them.
7 Only be thou strong and very courageous, that thou mayest observe to do according to all the law, which Moses my servant commanded thee: turn not from it to the right hand or to the left, that thou mayest prosper withersoever thou goest.
This book of the law shall not depart out of thy mouth; but thou shalt meditate therein day and night, that thou mayest observe to do according to all that is written therein: for then thou shalt make thy way prosperous, and then thou shalt have good success.
Have not I commanded thee? Be strong and of a good courage; be not afraid, neither be thou dismayed: for the Lord thy God is with thee whithersoever thou goest.