Consistency is key so be sure to weigh each meal out 3-4 days in advanced. This will eliminate the urge to binge or mess up your macros.
Getting the right amount of protein into your daily diet will help you maintain muscle mass throughout prep. I use Maximuscle Whey Protein Isolate, BCAAs intra workout and Creatine Monohydrate post workout.
3. Proportion / aesthetics
Figure out your weaknesses / areas you need to improve on and make sure you work on them more than once a week. Proportion & symmetry are key.
4. Training partner
I found a having a training partner who is of similar strength has helped me tremendously throughout my prep. Hugely helpful on those low days, enabling me to get the most out of my workout.
5. Training programme
Get yourself a training programme to follow and make sure you log the weights you lift. This helps to track your progress as you will want to try and increase the weight you lift each session.
6. Don't try too hard on stage
When you finally reach the stage there's nothing worse than being the guy who is trying too hard. Try to look relaxed and make sure you keep your abs and legs squeezed. Most importantly SMILE!
Either find a good posing coach or someone who has competed before to help you with your routine, PRACTICE as much as you can before the show, when the adrenaline kicks in your mind can go blank so be prepared.