Romans 8:5 NKJV

 Romans 8:5 - For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit, the things of the Spirit.

To explain what's said here in more, and different, words, those who live according to the flesh have desires of the flesh that supersede desires to be holy and follow God. Living according to the flesh is seeking earthly desires first, earthly meaning human desires, not God's desires. See verse 7: the mind set on the flesh cannot please God because it cannot follow His law. It doesn't place God's law above its own law and desires.

The flesh and the Spirit are set opposed to each other here, which I think helps us to define living according to the Spirit. While not the only definition, we see in verse 7 that living for the flesh is living opposed to God's law. It does not submit to the law of God. Living in the Spirit, then, is submitting to the law of God (again, not the only definition). Submitting oneself and ones desires to God instead of to themselves and their earthly desires; that's one aspect of living with a mind set on the Spirit.

Now, it is very important to note that we are not capable of living a sinless life in this earthly, flesh-filled, fallen body. So by submitting oneself to the law I do not mean perfectly upholding it from this point on, because if I did, then none of us can do it. Rather, it's the understanding that the law of God supersedes all our wants and desires, especially the evil ones, and that when we fail to uphold the law we do not hide it from God but instead come before God in confession. We have Christ Jesus, our Great High Priest, interceding for us always, so when we fail and we repent, Jesus is up there interceding for us, showing the finished work on the cross for our salvation and the paid price for our sin.