C.S. Lewis cautioned against both imagining a demon behind every tree and believing Satan doesn't exist. While two kingdoms are indeed in conflict (Matthew 12:22-30), Jesus Christ, through His death on the cross, made a spectacle out of the powers of darkness (Colossians 2:15). What this means is that we engage in spiritual warfare by the sheer act of pursuing Christ.
Scripture admonishes us to stand firm against the devil and the evil forces of this world by employing the full armor of God, which is metaphorical language for knowing the truth, exercising faith, and practicing righteousness in our daily lives (Ephesians 6:10-18; cf. 2 Corinthians 10:4-6). To engage in spiritual warfare is to believe in Jesus Christ for the forgiveness of sins and to lead lives characterized by honesty, purity, prayer, Bible study, evangelism, and so on.
If someone struggles with bad thoughts, for example, rather than addressing demons on the issue, we ought to replace the bad thinking pattern with a good one. First, regardless of the source of the original thought (whether from Satan, another human, or our own sinful flesh), we are responsible for what we do with it. Furthermore, while we cannot directly stop thinking a bad thought (it's virtually impossible not to think about a pink elephant when told not to!), we can consciously focus on something else in its place — a thought that is true, noble, right, pure, lovely, and so forth (Philippians 4:8-9). Finally, practicing discipline in one area of our daily lives can affect our ability to overcome bad habits in other areas as well.
The bottom line is that we must submit ourselves to God through faith in Jesus Christ, who by His life, death, and resurrection has defeated Satan (Hebrews 2:14; 1 John 3:8). What remains is to simply resist the devil, for then, the Bible says, he will flee (James 4:7).