"Resist the devil, and he will flee from you." James 4. 7
Someone has said that the devil's greatest asset is that nobody believe in him anymore. That is alarming because Paul reminds us that "our struggle is not against flesh and blood,
but... against the spiritual forces of evil in the heavenly realms" (Ephesians 6. 12). Chief among these "spiritual forces" is the devil himself: Satan, the great accuser - so called because of his fondness for mis-representing us before God.
How can we fight if we do not acknowledge our opponent? Jesus certainly acknowledged the devil and made dealing with him his first priority, facing him down in the wilderness immediately after his baptism (Matthew 4. 1-11). Though he may be defeated, Satan isn't finished yet. We know this for three reasons: 1. Jesus repeatedly had to drive him out during his earthly ministry; 2. The apostles continue to urge caution and awareness of the devil and his wiles in their letters; 3. John looks forward to the devil's ultimate come-uppance occurring only when Jesus returns "at the end of the age" (Revelation 20. 10).
In the meantime we need to be ready and equipped to resist. The encouraging thing is the implication in James' statement above that, when we do this, he will flee - just as he did when Jesus resisted him in the wilderness (and just as he didn't when Adam and Eve notoriously failed in the Garden of Eden). So how do we "resist the devil"?
The most important thing to say is: not in our own strength. The sentence immediately before the one quoted states: "Submit to God..." because, in so doing we shall make sure we are on the right side of the struggle. Jesus warns "you cannot serve two masters..." (Matthew 6. 24). As Job learned through his ordeal, submitting to God does not spare one from suffering but it does ensure that we prevail in the end.
God is in control and the final victory is in his hands. The battle is not over but he will give us what we need to prevail, so we won't end up as collateral damage or, worse, as traitors who sell out - like Judas. If you like practical steps, a good place to start is by consciously and regularlly availing yourself of what Paul describes as the "full armour of God" in his letter to the church at Ephesus (chapter 6, verses 10-18). Check it out!