Everytime you feed the flesh, you strenghten it. This is good if you are trying to build a good habit, but detrimental if you are trying to stop a bad habit. The way to "kill the flesh" is to starve it; to stop feeding it.
Romans 6 - Our flesh is the instrument that Satan works through. When we say no to the flesh, we say no to the devil.
The flesh, or human nature, is generally lazy and self-centered.