Train hard - Your body will not change unless it needs to, you need to force it to change. Prioritise developing muscle tissue in the gym, the more muscle you have the faster your metabolic rate as your body will need to fuel that muscle.
Avoid drinking too much alcohol - Okay I didn't want to put this on here, just because I like to focus on what you can add into your training and nutrition, and not what you should take away. However, I do often look at peoples weekly diet and think to myself "If you didn't consume 900 calories worth of alcohol (5 pints), followed by the a 1,200 calorie kabab, followed by a 1,335 KFC (Big Daddy Box Meal) the next day (hang over food of course), if it wasn't for all that, I would be looking at a good week".
Track progress - Unless you're increasing volume, intensity or frequency over time (progressive overload) you will hit a plateau with your progress, you'll need to track progress to make sure you're consistently developing.
Control calories - You don't need to starve yourself, you simply need to be burning more calories than you're consuming.
Rest and recover - You need to give your body time to develop and recover, 7 to 8 hours sleep is ideal.