Keep track of your expenses and ensure they are close to your estimates. If you change jobs or get a pay rise, revise your income figures.
Every few months or so, it's a good idea to go through your budget and your plans from scratch. Adjust your budget if you have to.
Keep in mind that you may occasionally be hit with an unexpected bill, such as car repairs or a parking fine. So if you blow your budget in one period, see if you can make a temporary adjustment in the next period to get things back on track.