I never thought I wanted to be a cowboy, but as it turns out, I do!
We are instructed in 2 Corinthians 10 to "[bring] every thought into captivity to the obedience of Christ." The word used for captivity in this passage means to lead away as a prisoner. This is similar in context to a cowboy riding a horse and lassoing his prey.
When a cowboy swings his lasso around in the air, his focus is on his prey. He reaches high into the heavens with his hand outstretched, concentrating, focusing on the cattle in front of him. Then he swings his arm around one last time and lets the rope go, lasso falling on the animal in his sight. Once he "ropes" the animal he jumps off his horse and ties the prey up, wrapping him by the feet so he can no longer move around and have free reign. He is held captive by this cowboy.
So it is with us in the battlefield of our mind. We are to take our thoughts that are unpleasing to the Lord, lasso them, tie them up completely, and stop them from roaming free in our minds. Only then can we be victorious and walk this race with endurance, the race that God has set before us. We can be victorious!