Bad habits? That is a profound and useful question. Hope you get lots of responses.
1. One could do worse than starting off by reading a chapter or two in Benjamin Franklin's Autobiography. (Free download at Gutenberg.)
2. Is it fair to begin by discussing how NOT to handle bad habits: don't wake up in the morning and start nagging yourself with "Thou Shalt Nots." That just makes you want to rebel. The more you say, "I shall eat less/spend less/drive less this week," the more you are thinking about eating, spending, and driving. That means you have already lost.
Instead, stay busy with interests/obsession/ or hell, even new vices, so that you crowd the old vices out.
3. Never take the slightest step backwards when breaking an old bad habit; it will undo a month of positive progress. Visualize a ratchet wrench.
4. Moderately or gradually reduce weak bad habits.
5. Absolutely ban from your life strong bad habits.