Discuss entering a physique contest with even the most hardened gym veteran and you will begin to hear a whole host of excuses. They can't fathom standing up in front of people in speedos, etc. The excuses are just that–excuses. I personally believe that a physique competition is something you should consider if your goal is to get into top shape. It is motivational and pushes you to do your best.
When I needed to get the weight off and build muscle I set a deadline (a show date) and then advertised it to every single person who would listen. There was no way I was pulling out. It got me focused and gave me a reason to suffer through some of the tough times you go through when getting into great shape. If you are thinking about doing this, here is the approach I recommend:
The Goal. Set a goal by looking at where you want to be and how long you think it will take to get there. Now look for a show that is in your area around the time of the goal completion. I recommend not going past your goal date, but a couple weeks earlier is OK.
Advertise. Once the goal is set make sure to tell every single person you know. Explain to them that you are going on stage no matter what. If you don't meet your goals you will go up and be humiliated. If you meet your goals you will go up and feel great about yourself.
Ride the Accountability. After you advertise you are going to have people asking about your progress all the time. Ride that accountability wave to success. Take advantage of the fact that you have created so many accountability partners. Use that to your advantage and your success will be virtually guaranteed.
Watch my rant on competing: