[JessR]

The major overheating and motor damage scenario is when you are running full current through the stalled motors in a shoving match. Then none of the power is converted to motion, it all turns into heat in the motor armature.
You can do the math - average specific heat of say 8 ounces of copper and steel motor parts, heated by 40 to 200Watts continuous power input. It heats quickly.
So a key is to train drivers to avoid shoving matches wherever possible, and keep them short when they do occur.
That said, anything which increases the rate of heat transfer from the motor body to the ouside air helps keep the average and peak motor temperatures down.
Its all about keeping up the highest 'delta T' between the motor surfaces and the air next to them, to maximize heat transfer rate into the air. Fans are great because they continuously displace the warm air next to the surfaces and replace with cooler ambient air. Heat sinks help because there's more surface area for heat to conduct into (aluminum is a much better conductor than air = higher heat transfer rate away from center of motor) and gives more surface area for air to contact = more air mass incontact to transfer heat into.
I love the coke can idea, it is elegant and simple.
Jesse.