Lots of great advice given here. As for me, I travel regularly and have not had a problem yet. However, I always take the following steps, which greatly reduce the odds of having to check a camera bag:
1. First and foremost, make sure the size meets the carry-on requirements. (They rarely check the weight.)
2. I always use a non-descript color such as black. It draws less attention. (I own a black Pelican 1514 and a black Think Tank International.)
3. I always approach the gate agent with the carry-on bag on the opposite side from where the gate agent is standing. For example, if the gate agent will be on my left-hand side when I approach him/her, then my roller is pulled by my right hand. To some degree, your body blocks their view of the carry-on bag and will make it less likely for them to "investigate" what type of bag you're carrying on board. Never ever give the impression that it's heavy.
4. As I approach the gate agent, I always start out with a hello, a courteous smile, and ask them how they are doing today. By engaging them with friendly conversation, it's less likely they will say something to you. It also chews up most of the normal contact time they have with you, and they will want to move on to the next person in line. And lastly, why not use this approach anyway and brighten someone's day?
5. I haven't had to do this yet, but if a gate agent tags your carry-on and you know it will still fit, then I will take the tag off while walking down the jet-way to board the aircraft.
6. It's been mentioned already, but I never book a seat at the front of the aircraft as those folks may be out of luck if the overhead bins get too full.
Follow these guidelines and you'll probably eliminate 98% of the situations that would cause you to check your carry-on bag.
Hope this helps,
Bryan