I don't want to sound callous, but boarding the plane early, and thus insuring your carry on makes it on board, is one reason why many flyers strive to attain status with an airline. If gate agents let people board out of turn (unless they are disabled, infant, etc.), there goes one major perk of loyalty to a particular airline.

I tend to fly *A carriers, so I don't usually have a problem with carry ons not making it onboard. Sometimes when I have to wait at the gate for an upgrade, I know there won't be room once I get on - but the FA is usually kind enough to allow me to store in the F locker.

When I can not board early (i.e first 1/3 of passengers) with a carrier I don't normally fly on, I usually ask the agent who boards first (rear, front, window) and try to get into that group. This is the case even if it is a lousy middle seat in rear, since E- is all the same really.