“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.”  
Joshua 1:8

It is always awesome when we find out through the word of God what our responsibilities are to succeed in life and walk in righteousness.
For example, to be saved, the unbeliever ought to:
“That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.”  Romans 10:9
It’s that simple, nothing extra- that’s the responsibility of the unbeliever, and after he’s saved then he goes on to tell others:
“… and hath committed unto us the word of reconciliation. Now then we are ambassadors for Christ” 2 Corinthians 5:19-20
That’s the responsibility of the man who is saved.
In the main verse, he says “for then thou shalt make thy way prosperous”, this is quite different from what is popular, one would think it is God that will make the way of an individual prosperous, but the word of God places that responsibility on the individual. This is good news! Knowing that there’s something you can do to make your way prosperous means your prosperity is now a choice, and you can increase or enlarge it based on the frequency with which you do what God said you should. He tells you how: “This book of the law shall not depart out of thy mouth”
He says talk the word- thinking about God’s word is good, but you want to prosper you must learn to talk the word- keep it in your mouth! Jesus said:
“…he shall have whatsoever he saith.”  Mark 11:23